Some useful formulae

Average time in the queue $=\frac{}{(-)},$ Average time in the system $=\frac{1}{-}$

Expected queue length $=\frac{{}^2}{(-)},$ Average number of units in the system $=\frac{}{-}$

Probability that there are n units in the system at a particular time is

$P_n=(1-){}^n=(1-\frac{}{})(\frac{}{}{)}^n$