Bill was a bright student who excelled at arithmetic. One day, on his way to school, he passed a fruit store with a sign in the window: "Apples — Six for five cents." An idea struck him, and he went inside.
"How much are the apples?" he asked.
"Six for five cents," replied the shopkeeper.
"But I don't want six apples."
"How many do you want, then?"
"It's not about how many I want," said Bill. "It's a problem in arithmetic."
"What do you mean?" asked the man, puzzled.
Bill explained: "If six apples are worth five cents, then five apples are worth four cents, four apples are worth three cents, three apples are worth two cents, two apples are worth one cent, and one apple is worth nothing. I only want one apple. If it's worth nothing, I don't need to pay you."
With that, Bill picked out a good apple, began eating it, and walked happily out of the store. The shopkeeper was so surprised he couldn't say a word.