4 . A motorcyclist starts from rest at a point S on a straight race track. He moves with constant acceleration for 15 s, reaching a velocity of 30 m s-!. He then travels at a constant velocity of
30 ms-l for T seconds. Finally he decelerates at a constant rate coming to rest at a point F,
25 s after he begins to decelerate.

a . Sketch a speed-time graph to illustrate the motion.
b . Given that the distance between S and Fis 2.4 km, calculate the time the motorcyclist takes to travel from S to F.