A professor has 5 keys. One of the keys opens the door to his apartment. One night, he arrives at his building, reaches into his pocket and selects a key at random from those on his chain.He tries it in the lock. If it doesn't work, he replaces the key and again selects at random from the five keys until he finds his key, then stops. Let X = the number of attempts the professor makes. What is the probability distribution of X?