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# This has been reposted here to Lemmy as part of the "Curated Tumblr Project."
I made the icon using multiple creative commons svg resources, the banner is this.
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."
This picture just describes the 'new math' that everyone bemoans.
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.
9+7=10+7-1=16
Me, bad at math: yeah they taught us that as a way to do in grade school.
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.
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).
I like the way the second ADHD method is also a flip on the "seven ate nine" dad joke
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.