The average length of time that a customer waits in line for service is $W(x, y)=\frac{1}{x-y}, \quad x>y$
where $y$ is the average arrival rate, written as the number of customers per unit of time, and $x$ is the average service rate, written in the same units. Evaluate each of the following.
(a) $W(15,9)$
(b) $W(15,13)$
(c) $W(12,7)$
(d) $W(5,2)$