A corner grocery store has one cash register. The grocery store can service $20$ ?customers per hour. Customers arrive at the store at an average of $15$ ?per hour. Assume arrivals follow the Poisson distribution and service times follow the exponential distribution.

What is a customer $$ s average time in the system?

?

?a $.$ ?

$3$ ?minutes

?b $.$ ?

$12$ ?minutes

?c $.$ ?

$6$ ?minutes

?d $.$ ?

$9$ ?minutes