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. What is the average number of customers in the food-truck?