Problem 4. (15 pts) You are planning a bank. You plan for six tellers. Tellers take 15 minutes per customer with a standard deviation of 8 minutes. Customers arrive one every three minutes according to an exponential distribution (recall that the standard deviation is equal to the mean). Every customer that arrives eventually gets serviced. a. On average, how many customers would be waiting in line? b. On average, how long would a customer spend in the bank