This picture just describes the 'new math' that everyone bemoans.
this post was submitted on 18 Oct 2024
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I explained to a teacher one time this as my method, the get to ten version, and she looked confused as hell like why would anyone do that. She was cool with it though, gave me a whatever works for you kind of response.
This kind of feels like how I constantly get the "which word/shape/number (etc.) in this series is incorrect" questions on tests wrong. I severely overthink it. "Well, these four all have chloroplasts and this other one gains energy from photosynthesis via a symbiotic relationship with another organism, so it must be that one."
*Gets test back*
"Oh, it was the one that didn't live in a rain forest."
I like the way the second ADHD method is also a flip on the "seven ate nine" dad joke
Okay this is nice and all but how do people do 3974* 438 mentally, without paper? And bigger and some outright freaks seem to do it in an instant
Not any great easy way I can think of to do that one but I would attempt to do 400 by 3974 and then add chunks of 438 x 10 or x5 until I got really close and then add individual blocks.
So like 400 by 3974, you can round to 4000 and remove 4 x 26 = 104 after doubling 4000 twice. So we have 4000 to 8000 to 16000 remove 104 is 15896, add zeros is 1,589,600. Forget all other numbers but this one.
We are missing 38 x 3974. We can do the same round and remove trick to add 10 x 3974 by changing it to 10 x 4000 - 10 x 26. We need four of those though, so we can double it and turn from 40000 - 260 to 80000 - 520 and then 160,000 - 1040 or 158,960. Need to remove 2 x 3974 though, so remove 8000 and add 52 so 151,012.
Hopefully ive been able to keep that first number fresh in my head this whole time, which involves repeating it for me, and I'd add 1,589,600 and 151,012. Add 150000 and then 1,012 so 1,739,600 and then 1,740,612.
That all said, I make way more mistakes than a calculator, and I was off by 400 or so on my first run through. Also its really easy to forget big numbers like that for me. I'd say if you gave me ten of these to do mentally I'd get maybe 2 correct.
That's great but this is juggling numbers in memory and I simply cannot do this reliably. I will have this one current operation and put the other ones into the mental basket so to say and it evaporates and blurs as I calculate the other thing right so I wonder how these folks can do this and really fast. Not that I ever seriously tried other than some rare bored moments so maybe it is simply a matter of training?
Its very impressive though when you give these ppl two big numbers and they say result nearly in an instant
Over time those bigger numbers become more common too. Someone who can mentally do the type of problem I just did and get it right quickly likely have a ton of practice and will know quicker tricks, and be able to simplify it in a way.
Another part is they would be able to recognize a wrong answer more accurately as well. I didnt realize my answer was off by a lot until I put it in a calculator, but someone with more practice might know intuitively they were wrong.
I just don't consistently do this type of math, I used to be good at it in school but its become mostly irrelevant for me outside impressing someone a slight bit. It is helpful to have the ability to do things manually but it just rarely comes up.
For me:
3974 * 438 -> 4000 * 438 - 26*438 ->
4000 * 438 - 26*440 - 26*2 -> 4000 * 438 - 20*440 - 6*440 - 26*2
And so on, and I'd do some of the intermediate calculations as I go (e.g. 20*440 and 6*440).
But that's only really needed if I need a precise answer. If I can get away with an estimate, I'll simplify it even more:
4000 * 430 ~= 43 * 4 * 10000 = 86 * 2 + 10000 = 1,720,000
Actual answer: 1,740,612. 4000 * 440 would be easier (I like multiplying 4s), but I know it would overshoot, so I round one up and the other down. Close enough for something like estimating how much a large quantity of something kind of expensive would cost (i.e. if my company gave everyone a hot tub or something).
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9+7=10+7-1=16
I think that third one down is actually how they’re teaching it at my kids’ schools now. It’s called “making a 10” I think, basically that same idea, add up to 10 first, then do the rest.
Me, bad at math: yeah they taught us that as a way to do in grade school.