Distance between two stations A and B is 778km. A train covers the journey from A to B at 84km per hour and returns back to A with a uniform speed of 56km per hour. Find the average speed of train during the whole journey.

Explanation:-

$$\eqalign{ & {\text{Average}}\,{\text{speed}} \cr & = \left[ {\frac{{2xy}}{{\left( {x + y} \right)}}} \right]\,km/hr \cr & = \left[ {\frac{{\left( {2 \times 84 \times 56} \right)}}{{\left( {84 + 56} \right)}}} \right]\,km/hr \cr & = \left[ {\frac{{\left( {2 \times 84 \times 56} \right)}}{{140}}} \right]\,km/hr \cr & = 67.2\,km/hr \cr}$$