At the bank teller's window, arrivals and service times are randomly distributed (Poisson and exponential distributions, respectively). There is only one teller at the window. If the arrival rate is 10 customers per hour, and the service rate is 15 customers per hour, what is the average waiting time for a customer in the system, including waiting in line and being served? Give the result in minutes. (It is a single channel, single server, single phase, unlimited waiting space model).