A car, starting from rest, accelerates at the rate f through a distance s, then continues at constant speed for time t and then decelerates at the rate f/2 to come to rest. If the total distance traversed is 15 s, then
(a) s = 1/2 f t2
(b) s = 1/4 f t2
(c) s = f t
(d) s = 1/6 f t2