A Greek electrical appliance manufacturer has three trucks that its various departments, such as production, receiving, mould shop, and maintenance shop, use as needed. The users have expressed that they wait an average of about nine minutes before a truck arrives (travel time is negligible). A study' over 17 days (a day has 15 work hours) recorded 1,172 requests for truck service (randomly distributed over time, implying that the number of requests per hour was Poisson). The average service time was 20.3 minutes with a standard deviation of 10.6minutes.

a. Calculate the approximate average wait time in the queue for the truck users by using Model 7. 

b. Give some possible reasons for the actual wait time in the queue being so much larger than your answer to part a.