A bank uses s tellers to handle its walk-in business. All customers form a single line in front of the tellers and get served in a first-come-first-served fashion by the next available teller. Suppose the customer service times are iid Exp(u)random variables and the arrival process is a PP(y). The bank lobby can hold at most K customers, including those in service.

Suppose the arrival rate is 18 per hour, the mean service time is 10 minutes, and the lobby can handle 15 customers at one time. How many tellers should the bank use if the aim is to lose no more than 5% of the customers?