19. Telephone calls come into a 24-hour airline call center (handling calls worldwide) randomly at the mean rate of 15 calls per hour. The time between calls follows an exponential distribution with a mean of 4 minutes. When the two reservation agents are busy, a telephone message tells the caller that the call is important and to please wait on the line until the next reservation agent becomes available. The service time for each reservation agent is normally distributed with a mean of 4 minutes and a standard deviation of 1 minute. Use a two-server waiting line simulation model to evaluate this waiting line system. Simulate the operation of the call center for 800 customers. Discard the first 100 customers, and collect data over the next 700 customers. a. Compute the mean interarrival time and the mean service time. If your simulation model is operating correctly, both of these should have means of approximately 4 minutes. b. What is the mean customer waiting time for this system? c. Use the =COUNTIF function to determine the number of customers who have to wait for a reservation agent. What percentage of the customers have to wait?