Queuing Model The average amount of time that a customer waits in line for service is given by
$$
W(x, y)=\frac{1}{x-y}, \quad y<x
$$
where $y$ is the average arrival rate and $x$ is the average service rate $(x \text { and } y$ are measured in the number of customers per hour). Evaluate $W$ at each point.
$$
\begin{array}{llll}{\text { (a) }(15,10)} & {\text { (b) }(12,9)} & {\text { (c) }(12,6)} & {\text { (d) }(4,2)}\end{array}
$$