An airport limousine can accommodate up to four passengers on any one trip. The company will accept a maximum of six reservations for a trip, and a passenger must have a reserva- tion. From previous records, 20% of all those making reservations do not appear for the trip. Answer the following questions, assuming independence wherever appropriate.

a. If six reservations are made, what is the probability that at least one individual with a reservation cannot be accommodated on the trip?

b. If six reservations are made, what is the expected num- ber of available places when the limousine departs?

c. Suppose the probability distribution of the number of reservations made is given in the accompanying table. Number of reservations Probability 3 4 5 6 .1 23 .4 Let X denote the number of passengers on a randomly selected trip. Obtain the probability mass function of X.