A man started his journey from Lucknow to Kolkata, which is 200 km, at the speed of 40 kmph then he went to Banglore which is 300 km, at the speed of 20 kmph. Further he went to Ahmedabad which is 500 km, at the speed of 10 kmph. The average speed of the man is :

Explanation:-

$$\eqalign{ & {\text{Average}}\,{\text{speed}}, \cr & = \left( {\frac{{{\text{Total}}\,{\text{Distance}}}}{{{\text{Total}}\,{\text{time}}}}} \right) \cr & = \left[ {\frac{{\left( {200 + 300 + 500} \right)}}{{\left\{ {\left( {\frac{{200}}{{40}}} \right) + \left( {\frac{{300}}{{20}}} \right) + \left( {\frac{{500}}{{10}}} \right)} \right\}}}} \right] \cr & = \frac{{1000}}{{70}} \cr & = 14\left( {\frac{2}{7}} \right)\,kmph \cr}$$