Let’s make that 9 a 10 because it’s good enough, it’s smart enough, and goshdarnit people like it. Also, I don’t wanna add with a 9. So 10 + 7 would be 17, but we added 1 to the 9 to make it 10 so now we take 1 away, 17 - 1 = 16.
I just memorized any addition with 9 adds a 1 in front while reducing the other number by one. Same general step, but there’s no 10 in my head, just 9+7 -> 16. Basically, promote the tens column while demoting the ones column. I think of it more like a mechanical scoreboard (flip one up, flip the other down) than an operation involving a 10.
If it’s anything other than 9, I fall back to rote memorization, unless the number is big, in which case I’ll do the rounding to a multiple/power of 10.
Let’s make that 9 a 10 because it’s good enough, it’s smart enough, and goshdarnit people like it. Also, I don’t wanna add with a 9. So 10 + 7 would be 17, but we added 1 to the 9 to make it 10 so now we take 1 away, 17 - 1 = 16.
ezpz
9 plus a number? No. 10 plus a number, minus 1. Yis.
I just memorized any addition with 9 adds a 1 in front while reducing the other number by one. Same general step, but there’s no 10 in my head, just 9+7 -> 16. Basically, promote the tens column while demoting the ones column. I think of it more like a mechanical scoreboard (flip one up, flip the other down) than an operation involving a 10.
If it’s anything other than 9, I fall back to rote memorization, unless the number is big, in which case I’ll do the rounding to a multiple/power of 10.