Gnu Math Making Change

My lunch today cost $4.23. The young man working as the cashier saw the 5-dollar bill and rang up the sale, but failed to notice the quarter along with the 5-dollar bill. "I can't do that", he said, "I already rang it up." Now of course what he was actually saying was, "I can't do the making-change math in my head." But the Phactor knows a teachable moment was present. "OK, I just gave you 25 cents for a bill of 23 cents; how much do you owe me in change?" Blank stare. "2 cents", says the manager. "Now I just gave you 5 dollars for a 4 dollar bill; how much change do you give me?" Panicked stare. "One dollar", says the manager. "But I already rang it up", said the young cashier. "It comes out the same", says the manager. But you have to know when you're defeated. So after taking back the quarter, the change came to 77 cents. Relief swept across his face. "Are you a business major?" "No, communications." That was my second guess.