The driver of a car which is travelling at a speed of 80 km/h, locates a bus 80 m ahead of him travelling in the same direction.  After 15 seconds, he finds that the bus is 40 m behind the car.  What is the speed of the bus (in km/h)?

46.2 51.2  44.2 55.4

51.2

Let speed of bus = S 120 m = (80 - S) × \(\frac{5}{18}\) × 15 ⇒ \(\frac{144}{5}\) = (80 - S) ⇒ 28.8 = (80 - S) ⇒ S = 80 - 28.8 = 51.2