2. A supermarket is trying to decide how many cash registers to keep open. Suppose an average of 27 customers arrive each hour, and the average checkout time for a customer is 2 minutes. Interarrival times and service times are exponential. It costs $10 per hour to operate a cash register, and a cost of $80 is assessed for each hour the customer spends in the cash register area. How many cash registers should the store open? (40 points)