Problem $15-31($Algorithmic$)$

signal; no waiting is allowed. Each representative can accommodate an average of $11$ calls per hour. The arrival rate is $22$ calls per hour.

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

b$.$ What is the probability that a call will receive a busy signal if your recommendation in part $($a$)$ is used? If required, round your answer to four decimal places.