The driver of a car, which is travelling at a speed of 75 km/hr locates a bus 80 m ahead of him, travelling in the same direction. After 15 seconds, he finds the bus is 40 m behind the car. What is the speed of the bus (in km/hr)?

Let speed of the bus = x
Relative speed in same direction = 75 - x
Total distance = 80 + 40 = 120 m
$$\eqalign{ & {\text{Speed}} = \frac{{{\text{Distance}}}}{{{\text{Time}}}} \cr & 75 - x = \frac{{120}}{{15}} \times \frac{{18}}{5} \cr & 75 - x = \frac{{144}}{5} \cr & 75 - x = 28.8 \cr & x = 75 - 28.8 \cr & x = 46.2{\text{ km/hr}} \cr} $$