My dad taught me to count back change as a young child. He would sit with me in the floor and say "You are the cashier, I bought something that costs XXX and paid with XXX. How much money do you have to give me back?" And I had to count it out.. change first, then dollars. He always said "If you don't know how to count your change, how will you know if you are getting cheated?". I've actually had to tell the cashier how much money to give me back when he/she would accidentally cash out the register without putting in how much I gave him/her. My kids are amazed that I don't grab a calculator when doing math. I always tell them that I can figure it out faster than I can get out the calculator, turn it on and use it. They have even tested me on it. They hate it because I always tell them the right answer before they work in out on the calculator.