Probability and statistics

Ten people get on an elevator on the first floor of a six story building. Assume that each of the 10 people get off at random at any one of the five upper floors.

a) What is the probability that three or more people get off at least one floor?

b) Assume that the ten people are actually four couples and two of the couples have one child each. Assume also that families get off at the same floor. What is the probability that three or more people get off at least one or more floors?

c) Again assume the ten people are four couples and two children. What is the probability that the two children get off on different floors?