One third of a certain journey is covered at the rate of 25 km/hr, one forth at the rate of 30 km/hr and the rest at 50 km/hr. The average speed for the whole journey is :

Let the total distance = 1200 km


$$\eqalign{ & {\text{Total time taken :}} \cr & = \frac{{400}}{{25}} + \frac{{300}}{{30}} + \frac{{500}}{{50}} \cr & = {\text{ }}16 + 10 + 10 \cr & = {\text{ 36 hours}} \cr & \therefore {\text{Average speed}} \cr & = \frac{{{\text{Total distance }}}}{{{\text{Total time}}}} \cr & = \frac{{1200}}{{36}} \cr & = 33\frac{1}{3}{\text{ km/hr}} \cr} $$