Suppose that each of N men at a party throws his hat into the center of the room. The hats are first mixed up, and then each man randomly selects a hat. What is the probability that a)none of the men selects his own hat? b) exactly k of the men select their own hats? It's difficult for me to think of this problem in the abstract...I always work better with actual numbers as opposed to variables. Any help explaining this would be appreciated! Thanks!!