A queuing model that follows the M$/$M$/1$ assumptions has $\backslash$lambda $=10$ and $\backslash$mu $=15.$ What is the average number of units waiting in the queue?

Group of answer choices

$0\mathrm{.}83$

$2$

$2\mathrm{.}5$

$5$

$1\mathrm{.}33$