This is interesting for a number of reasons. Probably my best practical learning in mathematics came while working at the concession stands in the local football stadium; I had to make change a lot. Here's how you do it: leave what they paid you easily video (so they don't later say "but didn't I give you a twenty?). Start with the amount owed. "That'll be $2.21." Count small change to even the number (in this case 4 cents to reach $2.25), large change to add up to an even dollar ("50... 75... 3 dollars), dollars to all to the total (5 dollars, 10 dollars, thank you). It's the opposite of the 'greedy' algorithm described by Miles Berry, and is what you actually do in practice.