A food-truck has one cashier and one cook. Customers first wait in line to ‘place the order and pay the cashier’, and then enter a ‘pick-up’ line to get the food that is being prepared by the cook. Currently, the cashier can take an order and handle the money transaction in an average of 1 minutes. The cook can prepare an order in exactly 2 minutes. During an hour, 15 customers arrive on average. The interarrival times are exponentially distributed.
After the training, what is the average length of the "place order and pay the cashier" line?