ionino's Pizzeria is considering opening a drive$-$through window for customer pickups. It is estimated that at peak times $(5-9$pm$),$ customers will arrive at a rate of $10\mathrm{.}5$ per hour. The clerk who will staff the window can serve customers at the rate of $1$ every $4$ minutes.

For the peak times, what is the average number of customers in the waiting line $($in the queue$)?$