No no no. Adding nine is just subtracting one, but adding to the front digit. 9 + 7 is actually 7 - 1=6, then add that 1 to the front. 16. Let's not make more complicated than it needs to be.
Holy shit! That's how I do it. Caught so much crap for it when I was a kid.
this is a false story. everybody knows 7 ate 9
I realized something. I relate so much to ADHD memes not because i have it but because they simply do a lot of things that they think only people with ADHD do. In my school they encouraged you to come up with techniques like this. Often 9 is hungry in different ways. Another exmple is multiplication. 5099 is 50100-50 which is much easier to calculate.
Yo but hear me out. Because 7 ate(8) 9, 7 + 9 = 7
9 is 3+3+3, 7+3 is 10, 3+3 is 6, 6+10 is 16. I'm also a fucking heathen.
What the fuck
Might as well do:
9 is 1+1+1+1+1+1+1+1+1, 7 is 1+1+1+1+1+1+1 therefore 9+7 is 1+1+1+1+1+1+1+1+1+1+1+1+1+1+1+1 which is 16.
That's pretty much exactly how you first learn to do arithmetic. Break the (whole) numbers down into their smallest (whole) part and count em up. That's what number lines do, or using colored blocks to visually do the addition, or any number of other techniques you use when you're first learning arithmetic. I mean, even just knowing 3 + 3 = 6 is memorization, a shortcut form of 1 + 1 + 1 + 1 + 1 + 1 = 6.
lol, it is pretty bizarre I know. I just know 9 breaks into 3+3+3 because it's a square number, and adding one of those 3s to 7 makes it 10, which is easier to add stuff to, then I just get rid of the remaining 3s by adding them to 6, then 10+6 is a very easy equation to intuitively add, because you just replace the "0" with "6" to get "16" and you're done.
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.
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.
Me, bad at math: yeah they taught us that as a way to do in grade school.
9+7=10+7-1=16
I like the way the second ADHD method is also a flip on the "seven ate nine" dad joke
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