From 3:00 P.M. to 8:00 P.M. the local Big-W Supermarket has a steady arrival of customers. Customers finish shopping and arrive at the checkout area at the rate of 80 per hour (Poisson distributed). It is assumed that when customers arrive at the cash registers they will divide themselves relatively evenly so that all the checkout lines are even. The average checkout time at a register is seven minutes (exponentially distributed). The store manager’s service goal is for customers to be out of the store within 12 minutes (on average) after they complete their shopping and arrive at the cash register. How many cash registers must the store have open in order to achieve the manager’s service goal?