David:

What kind of math are you using? He gave her $20.25 for a $4.10 bill and she eventually gave him $16.15 so how does he owe her 5 cents?

If you really want to confuse a retail clerk try paying for a 50 cent item with a $2.00 bill. Did this once and got change for a 5. Told the clerk that I thought she had made a mistake. She said "Oh, was that a 20" and began to pull more money out of the register. I told her to look at the bill I had given her again and returned the excess change she had given me. And they vote.