A man completes a certain journey by a car. If he covered 30% of the distance at the speed of 20kmph. 60% of the distance at 40km/h and the remaining of the distance at 10 kmph, his average speed is:

Explanation:-

$$\eqalign{ & {\text{Let the total distance be 100 km}}. \cr & {\text{Average speed}} \cr & = \frac{{{\text{total}}\,{\text{distance}}\,{\text{covered}}}}{{{\text{time}}\,{\text{taken}}}} \cr & = \frac{{100}}{{\left[ {\left( {\frac{{30}}{{20}}} \right) + \left( {\frac{{60}}{{40}}} \right) + \left( {\frac{{10}}{{10}}} \right)} \right]}} \cr & = \frac{{100}}{{\left[ {\left( {\frac{3}{2}} \right) + \left( {\frac{3}{2}} \right) + \left( 1 \right)} \right]}} \cr & = \frac{{100}}{{\left[ {\frac{{\left( {3 + 3 + 2} \right)}}{2}} \right]}} \cr & = \frac{{\left( {100 \times 2} \right)}}{8} \cr & = 25\,kmph \cr}$$