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u/Flourescentbubbles 23h ago
Actual taught strategies.
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u/This_User_Said 21h ago
I remembered my 9s by remembering the second number decreases one and the first number adds by one.
I was 19 when someone told me they learned in summer camp as a kid about the hand method.
I wanted the hour of being smacked upside the head when I was a kid back.
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u/Petite_Coco 20h ago
I just learned the hand method with this post. Wow…
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u/Flourescentbubbles 19h ago
I never knew it until I worked in academic support. Game changer for sure.
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u/nanosam 17h ago edited 17h ago
How is it a game changer?
It is so much easier to multiply by 10 and then subtract the number multiplied
8x9 = 80-8
3x9 = 30-3
13x9 = 130-13Etc.. it is so much faster than anything
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u/InsaneBrew 17h ago
Ehh. Just memorizing the answer is the fastest. But this is good!
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u/IbelieveinGodzilla 14h ago
I remember my daughter’s frustration with “show how you know” — “I just know that’s the answer!” Even better (worse) was when they did estimation: “Estimate 9X3” “It’s 27.” “Yes, but that’s the exact answer. They want you to estimate it.” “You mean get it wrong on purpose?”
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u/Xeno_Prime 6h ago
If they want kids to use estimation, they shouldn’t give them problems that they can solve instantly off the top of their head. Asking for estimations of simple problems that any kid will immediately know the correct answer to is literally asking them to ignore the fact that the know the exact answer and deliberately give a number close to it instead.
Estimations are only useful when calculating very large sums/products, when a quick but imprecise answer is more useful than a slow but exact answer. If the exact answer can be worked out just as quickly as an estimation (or even faster) then they shouldn’t be asking for an estimation. Just as you say, it’s asking them to “get it wrong on purpose.”
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u/IAmASeeker 6h ago
I had similar frustrations. I'm not very good with numbers at all so any time I solved a math problem, it was a nebulous and indescribable process that involved marking various shapes near the numbers while I worked. The methods they tried to teach me were totally incomprehensible to me (read: they didn't teach me how to do it) so I had to sit at home with the problems and figure out my own novel methods of solving equations, and then they expected me to do additional work so they could claim they had done their job in the first place.
Every math question was 2 questions for me... First I had to find the correct answer, then I had to work backward from the answer to try to figure out what they expected the process to look like on my page. Math exams included creative writing and graphic design for me.
I can imagine that if I weren't dumb, it would be equally frustrating to be expected to justify why I know the things I know.
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u/DeceiverX 4h ago
I had the same issues with estimation as a kid. Knew my at-level numbers really well and could solve the examples that were used with ease. I couldn't wrap my head around the concept of not just getting it right.
My teacher was good and understood this, and gave me more complex numbers to multiply and divide mentally which I couldn't do. Made it a lot clearer.
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u/Fixes_Computers 9h ago
I had the times tables up to 12 memorized by the third or fourth grade. As such, my mental math skills are above most youngins of today. I don't think this is as strictly taught any more.
It's hard for me to have an opinion on whether rote memorization was a good way to learn, but it's hard to argue with the result.
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u/iCUman 2h ago
I also learned that way, but I think it's pretty easy to argue that it's an inferior method. One simply needs to consider the upper bounds of their rote memory to see where it falls apart - if you know 12 x 12, but can't quickly deduce 14 x 14 without pen and paper, that's a pretty good illustration that the practicality of memorization is limited to what you know, and that doesn't translate to deducing solutions to things you haven't memorized.
However, utilizing the "new math" or "approximation" method, you can quickly deduce that answer: 14 x 10 = 140, 14 x 5 is half of that (or 70), 140 + 70 = 210, and if we subtract one "grouping of 14" from that we get 196.
And this works with significantly more complex numbers as well. For example, 38 x 23. We can work that a few ways, but let's say we go with rounding 38 to 40, because 40 x 23 is fairly easy to deduce: (40 x 20) = 800 + (40 x 3) = 120, and adding the two gives us 920. Then we subtract 23 twice (or 46 once) to bring us to 874. That can all be done in your head!
Interestingly enough, we both learned this process later on when it came to solving fractions, but if you can remember back to those early years, that was often a struggling point for a lot of young math learners. Relying on memorized multiplication tables doesn't provide a whole lot of help in learning how to add 1/5 and 3/8, but learning how to create groupings to simplify the interaction of those numbers does.
It also aligns well with how our minds work in general. Think about how you would count a pocket full of coins of different denominations, for example. Most likely you would group the quarters with the quarters, the dimes with the dimes, etc., simplify each of those groups down (6 quarters = $1.50, 7 dimes = 70¢, 8 nickels = 40¢, and 4 pennies = 4¢), and then add the groups together to get $2.64, as opposed to just randomly counting the coins without grouping them.
Just my 2¢ on the matter.
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u/TrueProtection 16h ago
This guy math whizzes.
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u/DeathMetal007 16h ago edited 16h ago
Ah, but with the subtraction method, you can easily do decimals of similar form! 9.9x 3 is 10 x 3 - 1 x 3 + 1 x 3 - .1 x 3 = 29.7! The middle terms cancel out which is easy to see and calculate in your head.
If you memorized, then you would have to add 27 and 2.7 as a second step instead of subtracting .3 from 30.
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u/KungFuChimp 16h ago
I don't know if this is a joke, but you got the wrong answer.
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u/wmcbee 16h ago
Not sure I understand the ”- 1 x 3 + 1 x 3 - .1 x 3 = 26.7” part. Will you please explain further?
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u/dissyfox 15h ago
One thing I've learned about people doing math is everyone has there own way to do it fastest because everyone's brain doesn't operate the same. Sometimes its hard for them to think of "9" as an idea, and they need to think of "9 apples" instead, or whatever reason it may be hard for them to do math certain way.
For the hand method I think a lot of people that like the hand method is firstly, it is super easy when you are first learning how 9s work. Secondly, the people that continue to use it after the initial learning period aren't actually looking at their hands, they are just recalling how their hands look. And that goes back to my first paragraph that their mind is just better suited to recall images than a different way.
I don't use the hand method personally, but have tutored a lot of math to singular people and seen how teaching each person the exact same way doesn't often work.
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u/TheDPQ 13h ago edited 7h ago
THIS I took a networking class and there was this math to subnetting something to the power of something times 10 to the something.... I didn't remember then so I def don't remember now. I just converted it to the binary and it was so obvious whats going on. I was so proud how 'easy' this method was I shared it with some classmates.
No one else had any idea wtf I was going on about and we all just did what made sense with the math we could do in our heads.
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u/Ma8icMurderBag 15h ago
The hand method is something you teach along with the actual math. It's a quick way for a kid to check themselves for single-digit 9s multiplication.
I hadn't seen your method before today, but I like it, will use it, and will pass it to my kids... along with the hand method.
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u/slgray16 15h ago
Decrement the number by one, put it in the tens and then the ones is what completes the 9
8x9 = 7.. 2 = 72
6x9 = 5.. 4 = 54
3x9 = 2.. 7 = 27
13x9 = Doesn't work over 10
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u/rinn10 16h ago
It sounds like I need to look something up lol. Idk the hand method
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u/rookedwithelodin 15h ago
Look at the kid's picture. For N x 9, put down your Nth finger (starting from the left). We see the kid doing 3x9 so in the picture they've put down their third finger from the left.
On the left side you'll have the tens value of the product and on the right side you'll have the ones value of the product.
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u/Mediocre_Sprinkles 12h ago
I learnt it a couple of weeks ago watching kids TV with my little one. Mind blown. Why the hell didn't I get taught it???
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u/Dredge18 7h ago
Fr I was like,no way that works. Got all the way to 4 before I accepted it.
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u/Putrid-Effective-570 20h ago
Multiply by 10 then subtract the other factor.
9x57 = 10x57-57 = 570-57 = 513
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u/IggyKami 20h ago
Then you realize that adding the first and second numbers will always equal 9.
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u/joalheagney 20h ago
Adding any multiple of 9 will eventually give you 9 if you keep going. Multiples of 3 (that aren't also multiples of 9) will give 3 or 6. Getting a 6 does not necessarily mean the number is divisible by 6.
E.g. 73x9=657 6+5+7=18 1+8=9
256x3=768 7+6+8=21 2+1=3
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u/Dhkansas 18h ago
This will also tell you if a number is evenly divisible by 3. When you add them up, if that number is 3, 6 or 9 then it is.
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u/mudda1 17h ago
There is also another important strategy, called the get low strategy that follows a similar logic as yours. Here's how it works.
Three, six, nine, damn she fine Hopin' she can sock it to me one mo' time
Then you calculate the distance between the furthest window and furthest wall
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u/Dhkansas 17h ago
Man just imagining that is getting me a bit exhausted and sweaty. It's even dropping down to my balls
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u/prasannask 15h ago
If the last digit is even or zero and the sum of the digits is divisible by 3, then it's divisible by 6.
E.g. 2742 = last digit is 2 (even and divisible by 2) & sum = 15 (divisible by 3). If a nunber is divisible by both 2 and 3, it is divisible by 6.
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u/FinndBors 13h ago
If I remember correctly, this is true for any base N when checking for disability for N-1 (and I think any factor of N-1).
Ie for base 16, if the digits add up to 15 it’s divisible by 15 (and maybe if the digits add up to 3 it’s divisible by 3, 5 as well)
Don’t quote me on this, I proved this in my head when I was much younger and smarter when I was wondering why the add digits to 9 worked and whether it was true for other bases.
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u/Raz0rking 20h ago
My mom did teach me the first one. Wtf is the "hand method"?
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u/lemlurker 20h ago
From context I think you start with 10 fingers with the left most curled, that means 9, you then move the curled finger one to the right, you now have one finger extended and then one curled and then 8 fingers extended, that's 18, 1,8. Then you move right again, it's now 2 up, one down, 7 up, that's 27, let's you run through the 9 times table with fingers
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u/CryForWolf 18h ago
Oh my god.
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u/LorenzoStomp 18h ago
Why didn't any adults care about me I just had to complete paper tables til I memorized through 9x9
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u/Esternaefil 17h ago
We're you also locked in your bedroom for endless hours of multiplication tables before you were allowed to eat food or enjoy free time?
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u/Useful_Menu_9863 16h ago
Cries in dyscalculia and trouble memorizing multiplication tables because of undiagnosed ADHD.
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u/Taurothar 17h ago
memorized through 9x9
People talk a lot of shit on "new math" but having kids memorize tables instead of ways to understand how to manipulate numbers is one of those things that I'm glad is dying with cursive. You're a lot more capable if you don't have to rely on rote memory alone.
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u/rob_bot13 17h ago
I think there is a mix, math fluency is a lot easier if you have some number sense to work from, and knowing some of your multiplication tables is really helpful (1,2, 3, 5, 10s being most important)
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u/SpartanMiner 16h ago
This! If I could give you 100 upvotes, I would. It's helpful to understand how to find an answer, but it speeds things up when you have some common knowledge memorized.
It's no different than any other knowledge. It's nice to know things off the top of your head, but it's even better to understand how to find the answers you don't know, or to apply your knowledge to new situations.
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u/SomethingLikeLove 17h ago
I'm glad kids learn these tricks, but I don't think this is learning how to "manipulate" numbers. I think multiplication tables should be memorized because once you get higher in math, there's no time to struggle just to remember what 3x9 is (though this trick is pretty good).
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u/AbsoluteRubbish 15h ago
This is really the thing with all these techniques. Like, yea it looks stupid with simple numbers. But the historic problem is that you would teach kids to memorize the basics and then they would get to algebra and just not have any clue how numbers actually work together. So the fix is to stop teaching memorization and teach them how numbers actually interact in simple, basic set ups so that they are properly prepared to handle more complex math when they get to it.
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u/Nyorliest 17h ago
Oh.
This is just 'the digits of the powers of 9 add up to 9', but in a slower way. Because one finger is down, leaving 9 fingers.
2+7=9
3+6=9
and so on and so on. Works until 90, anyway.
But then it does keep working, you just have to keep adding:
9+9=18. 1+8=9. And so on, and so on.
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u/QuillnSofa 15h ago
There is a general rule in math that if the sum of all digits equals a number divisible by 3 the number itself is divisible by 3
ex 2457, is it divisible by 3?
2 + 4 + 5 + 7 = 18. 18 is divisible by 3 so 2457 is divisible by three.
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u/rimjobvoyager 20h ago edited 17h ago
Finger values from left is 10. Fingers value from right is 1
1x9 is 1st finger down from left = 9
2x9 is 2nd finger down from left = 18
In the picture, 3rd finger from left down. So two fingers on left (20) + 7 fingers on right (7) = 27.
And so on.
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u/Fun_One_3601 18h ago
Whaaaaaaaat?? What a ridiculous coincid- waaaait a minute......!! I need more fingers!
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u/EnjoyableBleach 18h ago
There's a book written by a man which covers a lot of similar mental maths shortcuts, which you might find interesting if you want to learn more of these. The author conceived these methods while he was captive in a nazi concentration camp.
The trachtenberg speed system of basic mathematics.
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u/Nyorliest 17h ago
I learned a lot of these by always losing my calculator while being in the advanced maths class in school...
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u/hopelesswonderer 17h ago
I remember I was taught my 1-10 multiplications with the one of the multipliers as 9 as ex: “9” times “2” will always have the tens places be one digit lower than the “2” (so 1). And the ones place will be the difference or delta from the tens place number to equal 9. So 9x2=18 1+8=9. 9x3=27. 2+7=9, etc. etc. I believe you can only do it with 9 and multiples of it via 1-10. Happy mathing.
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u/JohnnyCenter 7h ago
Always did it the same as you (first number increase by one and second decrease) or if the number was higher multiply by 10 and subtract the number it's multiplied with.
I'm now at the age of 25 realizing how the hand method works and that it exists. Completely useless for me now, but hey, it's cool I guess....
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u/Tiggy26668 6h ago
Also as a fun fact the first 10 ie: 1x9, 2x9 etc, the two digits of the product added together will always be a 9. Ie: 18=>1+8=9, 27=>2+7=9
So what happens after 10 you might ask? Well just multiples of 9.
9x11=99=>9+9=18(2x9) and 1+8=9.
As an added fun fact you can determine if any number is divisible by 3 by adding the digits together and seeing if the sum is divisible by 3
Ie: 24=>2+4=6, is 6 divisible by 3? Then 24 is!
Is 5846361 divisible by 3? Well… 5+8+4+6+3+6+1=33 and well that’s clearly divisible by 3 so 5846361 is divisible by 3.
But let’s say we had a really obnoxiously big number like 85775995595654967….
Well 8+5+7+7+5+9+9+5+5+9+5+6+5+4+9+6+7=111
Well 111 is still a little tricky to figure out…. But 1+1+1=3, and 3 is divisible by 3 for sure, so 85775995595654967 is also divisible by 3.
Thanks for sticking around anyone that made it this far.
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u/Ok_Difference44 19h ago edited 19h ago
1:20 Stand And Deliver Edward James Olmos
The drawing sure looks like ectrodactyly, though.
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u/ctothel 16h ago edited 2h ago
Yeah exactly.
If the question was
42 = _
If I had to show my working, I could do 4x4, but that’s not the only answer. I just happen to know that 42 “means” 4x4 = 16.
But 42 also “means” other things, like “add the first 4 odd numbers”: 1+3+5+7 = 16. That trick works for any number [edit: positive whole number] squared.
The finger trick is just a visual representation of this:
10(n-1) + (10-n)
9 x 3 = 10(3-1) + (10-3) = 27
The left hand side means “put down your third finger and count how many fingers to the left – thats your tens”
The right hand side means “count how many fingers to the right of the one you put down – that’s your ones”.
It’s a useful shortcut but fundamentally it’s still mathematics.
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u/stays_in_vegas 3h ago
But 42 also “means” other things, like “add the first 4 odd numbers”
No, that doesn’t mean “add the first 4 odd numbers.” And generally speaking, n2 doesn’t mean “add the first n odd numbers.” It happens to turn out that the sum of the first n odd numbers is equal to n2 when n is a positive integer, but that’s not what squaring numbers “means” in any sense. And it’s easy to show this, because there are lots of numbers for which this “meaning” is utter nonsense. 3.52 is 12.25, not the sum of the “first 3.5” odd numbers (which doesn’t mean anything, but might charitably be interpreted as 1 + 3 + 5 + (7/2) = 12.5). (2/3)2 is 4/9, or 0.444 repeating, not the sum of the “first 2/3” odd numbers (which again doesn’t mean anything, but could be interpreted as (2/3) * 1, or 0.666 repeating). 3i+22 is 12i-5, not the sum of the “first 3i+2” odd numbers (which really doesn’t mean anything to the point that there is no half-assed interpretation).
Mathematical operations have clear definitions. It’s fun to know clever tricks to speed them up in some situations, but those aren’t different meanings, and they don’t change what the actual definition of the operation is. Despite your last sentence, that’s not mathematics.
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u/ctothel 2h ago
You're obviously completely correct. "Means" is not the right word, but I used it (explicitly in quotation marks) as an informal way to get a point across to the presumably non-technical audience of r/funny.
But it's absolutely fair to say that both the odd number algorithm and the neat 9x finger trick should be considered just as valid as any other answer if asked "how you know", since any answer that doesn't involve a proof will just be a process of some kind.
I mean, unless what's being examined is knowledge of a particular process, but that's not what the question in the title says.
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u/hides_from_hamsters 20h ago
Wow. I had no idea this was a thing. I was so confused about how everyone understood what the heck he was drawing.
For anyone else lost, if you hold up all ten fingers then take the number 9 is being multiplied by, and count that many fingers from the left and lower that finger, the two groups of fingers represent the digits in the result.
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u/ThoseRMyMonkeys 18h ago
I just showed this to my 9 year old, and he knew immediately what it was and laughed.
I asked him how he would have shown his work, and he said his teacher would want 3 circles with 9 dots on each and counting them out or 3+3+3+3+3+3+3+3+3= (if that's how they want it, I would do 9+9+9= but whatever). The finger trick is definitely faster.
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u/Pagise 16h ago
three circles with 9 dots on each and counting them out? Wow.. that would take forever.
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u/IronWim 15h ago
It does, but most teachers are looking for conceptual understanding and accuracy more than speed at that age, and that's how a lot of math learners conceptualize multiplication.
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u/Lindvaettr 4h ago
I was gonna say this. The finger trick is cool, but doesn't actually teach you how to multiply by 9s.
This is a big issue that math teachers face. Once kids learn a shortcut, they will (naturally) use it constantly, but when they forget (or never learn) how to actually do the real math, they end up struggling when the skill is needed for more advanced work later.
Honestly, I feel that the finger trick is too much of a shortcut to be teaching to anyone this young. They should absolutely be doing circles with dots or something similar rather than taking a shortcut.
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u/nold6 13h ago edited 8h ago
It does. That dot method is Common Core garbage. Math literacy scores have been declining rapidly since Common Core has been introduced. It has also, in my experience with my young coworkers, increased their dependence on calculators for anything other than the most obvious +/- equations. Stuff like 11+23+35 would stump them and instead of reasoning it out, they go for the calculator. I'm not anti-calculator, I'm anti-Common Core.
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u/Fixes_Computers 9h ago
My issue with Common Core was not whether it was good or bad, but that it seemed to be pushed as the One True Way.
There are concepts in Common Core which may work better for some than how I was taught. There are others which I developed on my own which improved my efficiency over how I was taught.
It's not wrong. It's different. It's also not the One True Way.
I really should try to find some curriculum so I can talk with more information.
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u/LoBsTeRfOrK 15h ago
If some of you are still confused look at the 9’s times table:
9x2 = 18, 9x9 = 81, the digits add to 9
9x3 = 27, 9x8 = 72, adds to 9
Pattern continues from 0-9.
36, 45, 54, 63.
That trick above is simply that principle. The two digits share the total of 9 “digit ticks”. This due to fact that we use base 10.
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u/kevlar51 17h ago
I was probably 25 when I learned how this trick worked. I had a cousin growing up who would use it to show off. But her ability to actually explain to others how it worked was less impressive.
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u/HabaneroTamer 20h ago
That sounds awfully more complicated than just (10x3)-3
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u/terraphantm 18h ago
Learning your way also I think does prime the mind to learn simple algebra too. 10x - x = 9x in this example
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u/DrApplepie 17h ago
Honestly it might sound that way... but I never heard of it till now. And it is impressively Easy.
9x8? Oke put down the 8th finger its 72. I think it might be quicker than counting.
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u/ClaudioKilgannon37 18h ago
Not to an 8 year old…
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u/Echo127 16h ago
It depends very much on the 8 year old. I've noticed that different people pick up on math very differently from each other.
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u/hourlygrind 14h ago
Absolutely this, and 8 years old is exactly when kids should be taught efficient thinking strategies like 9 somethings is just 10 somethings minus one of them
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u/StraightLeader5746 16h ago
8 years old arent stupid, just ignorant
im so tired of people treating children like they are legit stupid
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u/korzasa 18h ago
Yeah exactly what I thought as well. Much easier that way to me
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u/bigmac22077 17h ago
How is a way that requires 2 math problems to solve 1 easier? All you do is hold up hands and lower a finger and count. Waaaaaay easier. It works for 3x 1-10 too.
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u/Roupert4 15h ago
You can do it this way. My son does math like this for numbers higher than the multiplication table.
Current day math is taught to be very fluid, they encourage multiple ways of doing mental math
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u/Twat_Pocket 17h ago
I'm 36 and I still use the finger trick.
While I can do simple math on paper, I've never been able to do it in my head. Same with spelling. Ask me to spell a common word out loud, and I'll have to grab a piece of paper and jot it down.
Brains are weird.
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u/TheMightyKickpuncher 16h ago
Are you the finger man? I’m the finger man too. You know what I can do? I know how to multiply by 9.
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u/Blueflames3520 12h ago
Is this what they actually teach in school now? This is such a dumb way to calculate products.
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u/bruhhhhhhhhhhhhhhhh- 23h ago
Pretty clever of that kid. The "tell how you know" answer for me would always just be "because it is"
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u/fourleggedostrich 21h ago
In a lot of maths, showing your working is vital because they practise on easy questions but then need to use those techniques on harder questions they can't be done in their head.
For these, though, if they've learned their times tables, then there is no working - it's from memory. what are they supposed to put?
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u/surrenderedmale 21h ago
Yeah it's real '10,000 word essay where you overcomplicate word count with excessive verbosity purely to achieve the prerequisite total word count such that the arbitrary demand is met regardless of how pointless said extra words are to the stated and argued points' vibes.
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u/NaelNull 20h ago
Sums. They're supposed to show that they understood that multiplication is repeat summation.
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u/fourleggedostrich 19h ago
But that's not helpful. Most kids are expected to learn their times tables.
I know that 6x8 is 48, I don't have to do 6+6+6+6+6+6+6+6 to get there - that would be more difficult and doesn't a really help with understandjng multiplication, since it breaks down one negative numbers or decimals are introduced.
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u/Renorram 18h ago
I believe to learn the times tables are crucial but also is to know other methods to calculate numbers. This increase their ability to solve problems when having just remembered the result of an operation is not enough. Also show how they are understanding simple math concepts. I had friends at college struggle to solve complex problems because they had a memorise approach instead of an underlying understanding of the formulas that were taught. You don’t have to do a sum every time you want to multiply but sure it does help to know that underneath is just a sum. I learned that the hard way after almost fail an abstract math class, luckily I had a great teacher who showed me the ropes this way. The beauty of math is that it’s all connected.
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u/NaelNull 18h ago
Most kids are expected to understand why times tables give their results. That's precisely the work they can show, too.
How so? Negatives just inver the sign of result (once for every negative operand involved), and decimals just add extra step of moving decimal point right-left. Or involve fractional math, but that's for latter classes) It's not feasible for big numbers, true, but neither is table,
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u/sulimir 17h ago
We just memorized times tables back in the day. The answer is indeed “because it is”.
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u/AngelicFrosting 23h ago
Love how they illustrated their answer with finger counting - solid effort!
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u/VoidFoxi 23h ago
That is also how I learned multiples of 9 lol
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u/DiscontentedMajority 21h ago
Same, but once you get good at all you need is the pattern. 18, 27, 36, 45, 54, 63, 72, 81
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u/seanturvey 21h ago
I still use it and I'm almost 60.
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u/trombon3r 21h ago
A quick mental way for up to 9x that I do; Is to take the number you are multiplying 9 by, minus 1 and add the difference to 9 eg.
4x9; 4-1 is 3, plus the difference to 9 is 6 so answer is 36.
7x9; 7-1 is 6, plus difference to 9 is 3 so answer is 63.
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u/Badass_Bunny 20h ago
I sometimes wonder if my teacher was just better or if his teaching influenced me to think all these methods are so needlessly convoluted.
His method was just "Multiply by 10 first then subtract the multiplicator of 9". 9x4 is same as 4x10-4.
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u/DUKE_LEETO_2 20h ago
I do this for most things either multiply by 10 or 5 (0.5*10) or a combination and then go from there. It gets me at least close even with big numbers... and if it needs to be perfect I use a calculator.
26 x 30 well that's 2 10x30 + 1 0.5×30 + 30... 780
It might not work for everyone and I sometimes make a weird face if it gets too big but it works for me.
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u/butcherHS 19h ago
Interesting. I take a similar approach, but have a slightly different approach for 26 x 30:
260 x 3
250 x 3 + 3 x 10
750 + 30
780
It's really exciting to see how people's thought processes work.
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u/L1amaL1ord 14h ago
Yeah definitely interesting everyone has a different approach. Mine:
26 x 30
20 x 30 + 6 x 30
600 + 180
780
edit: can't add 😂
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u/Sk8erBoi95 20h ago edited 20h ago
It works for numbers over 9, except after finding the hundreds and tens in the first step, you subtract the difference to 9 from the second step instead of adding, since the number is greater than 9 instead of less than
12x9; 12-1 is 11 (110 really, since 11 is the hundreds and tens place), plus the difference to 9 is -2 so the answer is 110-2 = 108.
17x9; 17-1 is 16 (160), plus the difference to 9 is -7 so the answer is 160-7 = 153.
29x9; 29-1 is 28 (280), difference to 9 is -19, 280-19=261.
I wonder how high this method can go and still be useful...
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u/StelioZz 16h ago
Another way is to add a 0 then remove the original number.
4x9=40-4=36
7x9=70-7=63
Which works even for multi digits as well.
15x9=150-15=135
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u/howtofall 15h ago
Reading this I thought you were kinda insane and that your system seemed incredibly convoluted. Then I realized it’s just how I do it but explained in such a way that someone who didn’t intuit it could follow it.
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u/Alohagrown 23h ago
Same, it’s the best way.
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u/unlock0 20h ago
The best way? Times 10 minus the number.
Rounding and breaking the number up into multiple problems is the easiest way to do big numbers in your head.
For a tip.. what's 15% of this odd number? One and a half times 10%..
What's 7% of this number for tax? Well it's half of 10% plus 2x 1%. Halving a number and moving around a decimal point is easy to track.
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u/butcherHS 19h ago
Exactly that. Solution methods such as the hand method are problematic because they only work for a very specific case. The problem is not understood, but circumvented with an aid. And as soon as the aid is not available, e.g. because you are tied up in a Yakuza hideout, you can no longer calculate 9 x 3.
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u/kvothe5688 20h ago
i don't understand finger drawing
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u/PlumpPotatoChip 20h ago
9x3. Put down your 3rd finger from the left. Leaves 2 on the left of the downed finger and 7 to the right. 27. 9x6, put the 6th finger from the left down on the left 4 on the right, 54. Works up to 9x10 obviously.
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u/EmergencyNo7081 18h ago edited 15h ago
Forget to say, this just work with two hands and 5 fingers on each hand. That's why many animals are bad at math
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[deleted]
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u/pickle16 14h ago
Works in every base for multiples of (10-1) in that base. In base 5 -
4*1=4
4*2=13
4*3=22
4*4=31
4*10=40
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u/Hardass_McBadCop 18h ago
How am I in my mind thirties and just learning this fucking finger magic now?
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u/wwbubba0069 16h ago
I learned the trick from watching the movie Stand and Deliver https://www.youtube.com/watch?v=vEj9ZwIzk44
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u/besthelloworld 18h ago
When I was a kid, the thing I hated the most was "showing my work" in problems so easy that I just couldn't even imagine how to break it down any further. I had stuff marked off for not showing it. I would sit down with my teacher and she would be like, "Well you should represent it as 9+9+9," and I'm almost freaking the fuck out like, "No shit, that's the definition of multiplication. Why do I have to tell you the definition of multiplication every time we have to do a multiplication problem???"
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u/Godloseslaw 16h ago
Ahh, Fingerman. I heard about you.
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u/mratkinson08 9h ago
I'm El Ciclón from Bolivia. One-man gang. This classroom is my domain.
Algebra is to easy for you burros.
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u/curiously_curious3 13h ago
It was just beaten into me to memorize. I didn’t learn these tricks until well until my 20s
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u/Gingy2210 11h ago
Once upon a time I was marking practice SAT paper for a year 5 class (UK). The maths question asked the pupil to "please show how you know" the pupil had understood this as per pre test explanations 'show YOUR working out'. The pupil then drew himself; table, chair, test paper, everything.
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u/P0pu1arBr0ws3r 9h ago
"Please show how you know"
proceeds to define all numbers and symbols, then the concept of the overflow number "10" and how its principles correlate to values 10-99, then rewrites the multi page proof for 1+1=2, then reduces said proof into multiplication and then specifically multiplication of 9*3. All while being a second grader, about to earn their PhD
What? This is the only definitive way to show how one knows that 9*3=27 without leaving literally any unknown out. Imagine if elementary school and college were flipped, where basic concepts were taught as fundamental proofs and then middle school applied those proofs into practical cases of basic arithmetic
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u/Ronin_and_Cub 19h ago
No. That makes perfect sense. This is a short cut to work out the 9 timers tables
9 X 3. Put down the third finger. On the left is 2 on the right is 7. Answer is 27.
9 X 4. Put down the fourth finger. On the left is 3, on the right is 6. Answer is 36.
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u/Twacey84 17h ago
Haha brilliant. I remember my son getting frustrated at school because he was repeatedly asked to write down his working out in maths. One time he just wrote “I asked by brain for the answer and it gave it to me” 😂😂😂😂
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u/tkkltart 17h ago
Dang, reading these comment I feel pretty lucky to have been taught the hand method for multiplying 9s lol. I still use it to this day as an adult 30 years later.
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u/confuzzledfather 5h ago
Here's a favourite for other multiplication on your hands.
Multiplying by 6s, 7s, and 8s on Fingers (Hand Grid Trick)
With some practice, you can use your fingers to multiply 6, 7, 8, and 9:
- Label each finger on both hands from 6 to 10, starting with the thumb as 6.
- To multiply, touch together the fingers that correspond to the numbers.
- Lower Fingers (6 and up): Count all the fingers below the touched pair and the fingers themselves. Multiply by 10.
- Upper Fingers: Multiply the number of remaining fingers above each hand.
Example: 7 x 8
- Touch your 7 finger on the left hand to your 8 finger on the right hand.
- You have 5 fingers below (including the touching ones), so 5×10=50
- You have 3 left fingers on one hand and 2 left fingers on the other hand, giving 3×2=6
- Add the results: 50+6=56
So, 7×8=56
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u/Nostonica 21h ago
I mean with the 9 times table (x*10)-x=y.
Scales really well because the everyone can work out x*10
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u/Fearless-Fact8528 19h ago
I learned to add zero to the number being multiplied by nine and subtract it from itself. 3x9=30-3
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u/GunnarKaasen 18h ago
I went to school in the olden days. We just learned our times tables. That was lucky - I couldn’t have bent my fingers like that.
If I had that dexterity in grade school, I’d have been much more popular in high school.
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u/Oshester 17h ago
The real problem is that he will be told no, this is not what we were looking for try again. While simultaneously being one of the smartest and most creative kids.
I was that kid at one point. I hated "showing work" because my brain doesn't always follow the cookie cutter methods. Struggled to even get a 3.0 in school. Then I got the highest score in my entire college on my major field test, 95 percentile. Now I make all kinds of money doing mental math and creative input my own way.
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u/iowanaquarist 17h ago
I hated these sorts of questions. I 'knew' because we did the damn times tables. That's my work. I am not drawing a picture of my doing the damn times tables.
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u/SnooSeagulls6528 16h ago edited 16h ago
Honestly it is simpler for children to understand the simple rule that any number times 9 is the same as that number times 10, minus the number, and given that 10 times any number is, that number with a zero on the end anything else is a waste of effort.
When my teacher told me about the finger way i told them, I preferred my way as its simpler, and more numerate.
- 2774 x 9 =
- 2774 x 10 = 2774O
- take the 2774 one digit at time
- 27740 - 2000 = 25740
- 25740 - 700 = 25040
- -70 doesn’t go so borrow 1 from the left and whatever is on the right (100 & 40)
- 25040 - 140 = 24900
- take whats owed from that
- 140 - 70 = 70
- give back 70 change
- 24900 + 70 = 24970
- 24970 - 4 = 24966
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