1. John and Bob take turns in flipping a fair coin. The first one to get a Heads wins. John starts the game. What is the probability that he loses?
2. Given a key pad which has nine numbers ranging from 1 to 9 on it. Suppose a valid password contains exactly 4 numbers. Use the inclusion-exclusion formula to compute the probability that a randomly chosen password contains at least one 5. Is there a simpler way to find this probability?