A supermarket stays open all night. The attendant in the single checkout aisle can service up to $250$ customers$/$hour$.$ One night, $220$ customers$/$hour arrive at the checkout aisle. $($Assume Poisson input and negative exponential service time.$)\%0$D$\%0$A$\%0$D$\%0$A $\%0$D$\%0$A$\%0$D$\%0$ADetermine the average number of customers in the system