A two$-$runway $($one runway dedicated to landing, one runway for dedicated to taking off$)$ airport

is being designed for propeller$-$driven aircraft. The average time to land an airplane is known to

be $1\mathrm{.}5$ minutes with a standard deviation of $0\mathrm{.}75$ minutes. Assume that the airplanes arrivals are

assumed to occur at random with exponentially distributed inter$-$arrival times. with average service

time of $1\mathrm{.}5$ minutes