¿Combinatorias o Permutaciones?

Question

Evans Hall on the Berkeley campus has 10 floors. Suppose the elevator starts at the ground floor with 5 passengers and each passenger is equally likely to get off at any one of the next 10 floors. Let X be the number of different floors that the passengers will get off. Find the probability P(X=k), for 1, 2, ... , 5, the expectation E(X), and the variance.