Basic Queuing Theory - Example

This video solve the following problem:
Arrivals of customers at a telephone booth follow the poisson distribution, with an average time of 10 minutes between one arrival and the next. The length of the phone call is assumed to be distributed exponentially with a mean of 3 minutes. Find:
a. The average number of persons waiting and making telephone calls
b. The average length of the queue that is formed from time to time
c. Probability that a customer arrive and find telephone booth is busy.
d. Probability that a customer arrive and find telephone booth is empty.