The Average Length of Time That a Customer Waits in Line

Multiple Choice

The average length of time that a customer waits in line for service is $W ( x , y ) = \frac { 1 } { x - y } , x > y$ 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 $W ( 18,6 ) .$ Note: $x$ and $y$ are given as the number of customers per hour.

A) $7 \mathrm {~min}$
B) $5 \mathrm {~min}$
C) $3 \mathrm {~min}$
D) $4 \mathrm {~min}$
E) $6 \mathrm {~min}$