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, 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 buying?