Question: The owner of a corner store stocks popsicles, gum, and candy bars. After school one day, he is swamped by an influx of 15 school children. They are in and out of his store in minutes. Later, the clerk reports that ten children purchased popsicles, seven purchased gum, twelve purchased candy bars, five purchased popsicles and gum, six purchased popsicles and candy bars, and two purchased gum and candy bars. The owner is very upset. Why?