The Friendly Neighbor Grocery Store has a single checkout stand with a full-time cashier. Customers arrive randomly at the stand at a mean rate of 30 per hour. The service-time distribution is exponential, with a mean of 1.5 minutes. This situation has resulted in occasional long lines and complaints from customers.
Because there is no room for a second checkout stand, the manager is considering the alternative of hiring another person to help the cashier by bagging the groceries. This help would reduce the expected time required to process a customer to 1 minute, but the distribution still would be exponential.
The manager would like to have the percentage of time that there are more than two customers at the checkout stand down below 25 percent. She also would like to have no more than 5 percent of the customers needing to wait at least five minutes before beginning service, or at least seven minutes before finishing service.
a) Use the Excel template for this model to check the current model. Also, find the probability that the waiting time before beginning service exceeds five minutes, and the probability that the waiting time before finishing service exceeds seven minutes.
b) Repeat part a to examine the alternative being considered by the manager
c) Which approach should the manager use to satisfy her criteria as closely as possible?