Matching Supply with Demand 3rd Edition by Gerard Cachon,Christian Terwiesch

Exercise 1

(Online Retailer) Customers send e-mails to a help desk of an online retailer every 2 minutes, on average, and the standard deviation of the interarrival time is alsO₂ minutes. The online retailer has three employees answering e-mails. It takes on average 4 minutes to write a response e-mail. The standard deviation of the processing times iS₂ minutes.
a. Estimate the average customer wait before being served.
b. How many e-mails would there be, on average, that have been submitted to the online retailer but not yet answered