If an item of mail cannot be delivered it is returned to the local depot and a card sent to the addressee inviting them to come to the depot to collect it. At the depot the one postal worker deals with customers arriving according to a Poisson distribution with a mean of ten per hour. Service times are exponentially distributed with a mean of 4 minutes.
(a) What is the probability that the postal worker will be idle?
(b) What is the probability that there are more than two customers in the queue?
(c) What is the mean length of the queue?
(d) What is the mean waiting time in the queue?