An observer has determined that the time headways between successive vehicles on a section of highway are exponentially distributed and that 60% of the headways between vehicles are 15 seconds or greater. If the observed decide to count traffic in 30-second time intervals, estimate the following:

a. The probability of counting exactly 4 vehicles in an interval.
b. The probability of counting 3 or more vehicles in an interval.
c. The probability of observing exactly 10 vehicles in two consecutive intervals.
d. The expected number of vehicles to experience time headway between 10?second and 20-second.
e. The average time headway of this traffic stream