Arrivals at a telephone booth are considered to be Poisson, with an average time of $10$ minutes between one arrival and the next. The length of a phone call is assumed to be distributed exponentially, with a mean of $3$ minutes. Find the following:

What is the probability that a person arriving at the booth will have to wait

$()$

What is the average length of the queues that form from time to time

$($ili$)$

Find the average number of units in the system