Sal$$s International Barbershop is a popular haircutting and styling salon near the campus of the University of New Orleans. Four barbers work full time and spend an average of $15$ minutes on each customer. Customers arrive all day long, at an average rate of $12$ per hour. When they enter, they take a number to wait for the first available barber. Arrivals tend to follow the Poisson distribution, and service times are exponentially distributed. What is the probability that the shop is empty? I have the arrival rate $=12,$ service rate $=4,$ number of servers $=4,$ but shouldnt arrival rate be less than service rate.