2. A supermarket is trying to decide how many cash registers to keep open. Suppose an average of 15 customers arrive each hour, and the average checkout time for a customer is 3 minutes. Interarrival times and service times are exponential. It costs $20 per hour to operate a cash register, and a cost of $0.25 is assessed for each minute the customer spends in the cash register area. How many cash registers should the store open? (30 points)