Quote Originally Posted by rubah View Post
I think the idea is that you start off using the abacus, and get so used to that, that one day you decide you don't need the abacus and do it from muscle memory?
I think a way to do it would be, if you have a good memory, to actually memorize all possible additions, somewhat like Allie says here.

It's how I do a lot of head calculations. I have a pretty good memory and if you've seen a lot of large number calculations, it becomes a something like: Hmmm, 563 times 146. Well, I know 553 times 145, so I'll just add 553 to that answer and 10*146.

It's a lot faster than actually doing all that in your head.