Customers walk into the local U.S. Post Office at an average rate of 20 per hour. On average, there are 5 people waiting in line to be served. The probability distributions that describe arrival and service times are unknown. The manager, Vicky Luo, wishes to determine how long customers are waiting in line.

APPROACH c Even though the probability distributions and even the number of servers are unknown, Vicky can use Little’s Law to quickly determine the average waiting time