Response to the following problem:
Jack's Joke Shop of Boston sells novelty items (joy buzzers, rubber chickens, whoopy cushions, and so on). Customers arrive according to a Poisson process on the average of once every 5 minutes. Jack prides himself on the personal service he offers his customers. The time Jack spends with customers follows an Erlangian distribution with a mean of 4 minutes and a standard deviation of 2 minutes.
a. What is the average number of customers in Jack's store at a given time?
b. On the average, how long does a customer spend in the store?