A small bank is trying to determine how many tellers to employ

A small bank is trying to determine how many tellers to
employ. The total cost of employing a teller is $100 per day, and a
teller can serve an average of 60 customers per day. On average, 50
customers arrive per day at the bank, and both service times and
interarrival times are exponentially distributes. If the delay cost per
customer day is $100, how many tellers should the bank hire?