Elmer Johnson went to the hardware store to make a purchase for his house. He asked the store clerk, "How much will one cost?"
The clerk thought for a moment and said, "Three dollars."
Elmer Johnson, who looked a little puzzled said, "Well then, how much will twelve cost?"
"Six dollars," replied the clerk.
Elmer Johnson scratched his head and said, "If I were to purchase two hundred, how much would that cost?"
"That," said the clerk, "will cost you nine dollars."
What was Elmer Johnson buying?