A fast$-$food restaurant has a drive$-$through window with a single server. Customers arrive at the rate of $20$ per hour and the service time is $2\mathrm{.}5$ minutes per customer. Determine the

a$)$ customer average wait time in line $($do not include the customer being served$).$

b$)$ average number of customers in line including the person being served.

c$)$ probability that $2$ customers are in line including the person being served.

d$)$ probability that the server is idle.

e$)$ Do you recommend adding one more server? Explain your reason.