18. Customers arrive at the rate of four per minute at a large bank branch in downtown Seattle. In its advertising, the bank stresses that customers will receive service promptly with little or no waiting.

(a) What is the probability that there will be more than 25 customers entering the bank in a 5-minute period?

(b) What is the expected number of customers that will enter the bank during a 5-minute period?

(c) If the bank staff can service 20 customer in a 5-

minute interval, what is the probability that the customer load will exceed capacity in a 5-minute interval, so that some customers will experience delays in obtaining service?