4. To deal with greater demand, Horton Williams Airport

(Problem 1) has opened up a second runway devoted just to planes taking off. Peak demand has now been bumped up to 15 planes per hour. Furthermore, each plane still takes about five minutes to position itself and take off, once it has been given the go-ahead.

a. (***) On average, how many planes will be waiting during peak time periods? How many will be in the system (waiting and on the runway)?

b. (***) How long, on average, will a plane have to wait before it is allowed to take off?