For a single$-$server, single$-$line, single$-$phase waiting line system, where $\backslash$lambda represents the mean arrival rate of customers and $\backslash$mu represents the mean service rate, what is the formula for the average time spent in the system?

Select answer from the options below

$\backslash$lambda $/\backslash$mu $(\backslash$mu $-\backslash$lambda $)$

$\backslash$lambda $2/\backslash$mu $(\backslash$mu $-\backslash$lambda $)$

$\backslash$lambda $/(\backslash$mu $-\backslash$lambda $)$

$\backslash$lambda $/\backslash$mu

$1/(\backslash$mu $-\backslash$lambda $)$