Yeah -- my mom wouldn't tell me my IQ until after I graduated from college. She didn't want me to figure I didn't need to work as hard in school. Truth is, I didn't, for many subjects -- but my social life is a constant struggle; there's always balance, of some sort...

I really like finding shortcuts, like the one I pointed out, the second method above. And some of them are simple, like when giving change: if the customer gives $10 for an item that costs $X.Y where Y is greater than zero, the bills to return are 10 - X - 1. (I also like thinking in variables, as it helps make my thoughts more generic so they can be applied in more situations.)

So for a more concrete example, the total is $3.69 out of $10. The bills part is easy: $10 - 3 is 7, then subtract one more to get 6. So, give the customer a 5 and a 1, and then some change: 100 - 69 = 31 cents.

I'm not sure I'm making sense, let me know if I've confused... :)