Problem $15-31($Algorithmic$)$

Mid$-$West Publishing Company publishes college textbooks. The company operates an $800$ telephone number whereby potential adopters can ask questions about forthcoming texts, request examination copies of texts, and place orders. Currently, two extension lines are used, with two representatives handling the telephone inquiries. Calls occurring when both extension lines are being used receive a busy signal; no waiting is allowed. Each representative can accommodate an average of $15$ calls per hour. The arrival rate is $30$ calls per hour.

How many extension lines should be used if the company wants to handle $90\%$ of the calls immediately?