A small call center, staffed by only one operator $($server$),$ has $20$ customers calling every hour with a standard deviation of inter$-$arrival times of $3$ minutes. $($This results in

$=1.)$ The activity time for a call center operator to process each call $($by addressing the caller$$s queries$)$ is$,$ on average, $2\mathrm{.}5$ minutes with a standard deviation of $8$ minutes $($or

$=10\mathrm{.}24),$ primarily because of the wide variety of caller queries. $($Hint: Consider these processing times to be General, not Exponential.$)$

Based on the information above, what is the average number of callers who are in the call center system $($either waiting or being served by the operator$)?$