Customers arrive at the rate of four per minute at a large bank branch in downtown Seattle. In its advertis­ ing, the bank stresses that customers will receive ser­ vice 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 pe­ riod?

b. What is the expected number of customers who will enter the bank during a five-minute period?

c. If the bank staff can service 20 customers 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?