A man completed a certain journey by a car. If he covered 30% of the distance at the speed of 20 km/hr, 60% of the distance at 40 km/hr and the remaining distance at 10 km/hr, his average speed for the whole journey was :

Let 10% of journey's = 40 km
Then, total journey = 400 kms
And, $${\text{Average speed }} = \frac{{{\text{Total distance }}}}{{{\text{Total time}}}}$$
$$\eqalign{ & 30\% {\text{ of journey}} = 400 \times \frac{{30}}{{100}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 120{\text{ km}} \cr & 60\% {\text{ of journey}} = 400 \times \frac{{60}}{{100}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 240{\text{ km}} \cr & 10\% {\text{ of journey}} = 400 \times \frac{{10}}{{100}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 40{\text{ km}} \cr & {\text{Average speed}} = \frac{{400}}{{\frac{{120}}{{20}} + \frac{{240}}{{40}} + \frac{{40}}{{10}}}} \cr & {\text{Average speed}} = \frac{{400}}{{\left( {6 + 6 + 4} \right)}} \cr & {\text{Average speed}} = \frac{{400}}{{16}} \cr & {\text{Average speed}} = 25{\text{ km/hr}} \cr} $$