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    Kevin and Peter love NIKE shoes. They like to wear them with T-shirts to school. One day, Peter tells Kevin, "Kevin, my sister works in a clothes shop. She says the NIKE shoes are on sale. We buy one pair of shoes and can get one for free!" Kevin says, "A pair of our favorite shoes is $216. So we can get one pair for $108 now, right?" "Yes," Peter says.

    Then they get to the shop and each takes a pair of their favorite shoes. The saleswoman gives them two pairs of socks and says, "They are $432.""Why? Don't you say "buy one and get one for free"? ""Yes, but one pair of shoes and get one pair of NIKE socks for free."