As I assume you know how to do it, carrying numbers in long addition is a short term memory thing. I bet you were roughly as bad at it when you were 17 as you a

As I assume you know how to do it, carrying numbers in long addition is a short term memory thing. I bet you were roughly as bad at it when you were 17 as you are now. Doing basic calculus in your head is way easier, it's simple multiplication.