Peter Winkler of Dartmouth College collects mathematical puzzles. Every once in a while, he encounters a particularly fiendish puzzle that gets him to scratch his head and wonder whether he heard it correctly. Or the puzzle sounds so trivial that he has to ask himself whether he missed something.
The following puzzle, which Winkler describes in the September College Mathematics Journal, belongs in the category: “Wait a minute—I must not have heard that correctly.”