A firm that prints affordable college and high textbooks to local schools here in South Florida. Kamal has a 800 toll-free telephone number that teachers, professors and librarians can call when thinking about adopting new books, or review copies of new materials, and also to place orders. Currently, Kamal has two extension lines to the number which are manned by two representatives who answer the phone inquiries. Calls occurring when both representatives are busy on the phone receive a busy signal, and there is no waiting allowed in the system. Each representative can accommodate an average of 12 calls per hour. The mean arrival rate is 20 calls per hour. Assume the calls follow Poisson arrival distribution and the service rate is exponentially distributed.
1. How many extension lines should be used if the company wants to handle 90% of the calls immediately?