10 + 7 = 17 17- 1 = 16
this post was submitted on 03 Oct 2025
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The legend said that it is how Gaussian elimination was discovered in europe
A dozen years ago or so there was a huge uproar about "common core" mathematics, which was a new standard being used in the USA for teaching.
It was a politicized trendy topic and even so-called-intellectuals were jumping on the train and calling it a deranged way of learning math.
I looked into it a bit, and I swear this pic pretty much sums up one of the key methods they were teaching.
Basically just tricks that a lot of people figure out to simplify problems.
Common core is still a thing. I wish I had common core as a kid. Makes way more sense.
That's exactly what it is. A way to help conceptualize and play with numbers. Stuff my bored ass was doing in school anyway before common core came around lol
9 is one less than 10, and 7 is three less than 10, so combined, they're four less than 20 = 16
Idk I have adhd and my working memory is so poor that memorizing time tables was the only way. :/
I thought you give 1 from 7 to 9 so it becomes 10+6 !!
You can do that, too.
I mean, sure, the choice of the "nice" numbers here is eccentric, but this is essentially the way math is taught nowadays. Only, instead of making 8 in this special case, the goal is usually to make 10 + leftovers because adding to 10 is always easy.
Here's my (upper midwest) spicy mental math take: it should be big-endian and solved with backtracking for ripple carry/borrow. None of this starting-from-the-1's-place-and-successively-incorporating-higher-order-digits nonsense. Extended carry/borrow is rare, and if you start with the most significant digits and give up/get bored part way through, the intermediate answer is in the ballpark of the real answer.
Why wouldn't you take the 1 from the 7 so it is 10+6?
This is what I go for...Just play in 10s. Alot easier IMO
NOT LIKE THAT YOU HEATHEN
No take one from 7 and its 10+6=16
Take 7 from 7 and its 16+0 =16
Take 11 from 7 and it is 20-4=16
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The answer is 69
420% of the time.
Niiiiice
And here I always thought it was 1001 + 0111 = 10000.
not even ADHD related you're just taking a route to something more readily available in your memory. that's how brains are supposed to work.
to me the detour is -1+10. whenever i see a 9 i take 1 away from the other guy and then add 10.
9 x single digit mumber works similarly; except i take away 1 and complete that to 9 by adding a number next to it.
9x7 = ?
7-1 = 6
6+? = 9
9x7 = 63
9x7 = 70 - 7 = 63 in table of 9 too easy ! (nearly the same technic)
8x7 = 70 - 7x2 = 70 - 14 = 6 + 70 - 20 = 56 (6 from 10-4 from 14)
7x6 = 5x7 + 7 = 70/2 +7 = 35 + 7 = 42 the answer to the life, the death and all the rest (5xa = 10/2 x a= 10a/2)
i mentioned the 5x trick elsewhere under this post but for me for some reason doing the halfing first is easier. so to me it's a/2x10 instead.
This is literally how common core math works.
This is psychopathy. Clearly you just add the 1 to the 9 to make it 10 and subtract it from the 7 to get 6 and 10+6 = 16.
That's the sort of thing "new math" was trying to teach. Those sorts of breakdowns are exactly what the kids who were good at math were always doing, and teaching methods eventually caught up and realized they should just teach the tricks.
Then a bunch of parents who were bad at math asked "new math? How can math change?" The fact that they even asked that question showed how their math education was lacking, but they seem to have won.
Exactly. Math has historically relied on rote memory for most mental math. Kids would have to fill out their times tables, addition tables, etc until they memorized them. I still remember getting pop quizzes in elementary school that looked like this:
You only had two minutes to fill out the entire thing, which meant you only had 1.2 seconds per answer. You didn’t have time to actually calculate them. The point was that you were expected to have them memorized ahead of time instead of calculating each one.
But rote memory is laughably bad at actually teaching concepts. You may know that 12x5 is 60, but you don’t have any understanding on why, or other ways to do that same calculation without rote memory. And rote memory is only decently reliable up to ~12x12. Anything past that, and it becomes too much info to track; kids simply start forgetting answers.
The kids who were good at math (and I mean actually good at math, not just good at memorizing things) quickly devised methods to do this shit in our heads easily. Keeping track of multiple numbers in your head gets confusing. So “line them all up, add straight down, and carry 1’s” sort of falls apart if you’re doing it in your head. Especially if you’re trying to keep track of more than three or four numbers at a time.
Essentially, 127+248+30 is the same as 105+250+50, but the latter is much easier to parse in your head. But yeah, the parents (who primarily relied on rote memory) didn’t understand why the new method would be more effective, because they didn’t understand the concepts surrounding the math.
i used to hate the times table but i definitely think it's essential to mental math. even if you vaguely remember it it will help. like knowing 42 shows up somewhere in the 7x and 6x may help you remember 6x7. or if you remember a neighbor you can just add or subtract the number once. for example if you don't remember 7x6 it definitely helps to know each neighbor (both of which are easier to me since one is a 5x and one is a square number)... so either you think about 7x5 which is 35 so you can add another 7 to it or 6x6 which is 36 so you can add another 6 to it.
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Why wouldn't you just take 1 from 7, add it to 9, and make it 10 + 6? That makes a lot more sense to my brain at least.
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I've always done it this way and don't have adhd
Holy shit.
Is that not how “normal” people do math in their head? How do they do it?
I have no idea how normal people do it but I do
9+7
One shy of 10+7
One shy of 17
16
How do they do it?
I assume they just don't. For my mom at least, she absolutely will not apply mental effort to anything that doesn't strictly require it. If a mental task can be offloaded to someone or something else, she'll do that instead, every time.
Borrow 1 from the 7 leaving you 10 and 6. This is what they tried to teach in schools for awhile but adults weren't getting it. Common Core? Is that what they called it?
One of my wifes friends was an elemetry school teacher when common core was popular. We asked her what it was and as she was explaining it, i said, "oh, like how you do mental math?"
Im an engineer and i just assumed thats how everyone did math... apparently people just memorized everything
As someone who learned not via Common Core, and then found out Common Core taught math how I taught myself to do mental math I was a little envious that kids would learn my “easier” method.
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My retarded ass: 9 is 0b1001 and 7 is 0b111, 0b1001+0b111 is 0b10000 which is 16.
Am kidding, I take 9's ten friend, sub-stract that from 7 et voilà I have 6 ones and 1 ten which is 16.
You’re so adhd you forgot that this was a whole part of your math curriculum that you just tuned out because you already knew it.
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9+anything
10+x-1
Similarly for 8+x is 10+x-2
Multiplying by 5: mult by 10 divide by 2 Mult by 15: mult by 10, add half of that.
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