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

The required average speed given by the formula,
$$\eqalign{ & {\text{Average speed}} \cr & = {\frac{{2xy}}{{ {x + y} }}} \,\,{\text{km/hr}} \cr & {\text{Where}}, \cr & x = 84{\kern 1pt} \,{\text{kmph}} \cr & y = 56\,{\kern 1pt} {\text{kmph}} \cr & {\text{Average speed}} \cr & = {\frac{{ {2 \times 84 \times 56} }}{{ {84 + 56} }}} \cr & = 67.2\,{\kern 1pt} {\text{kmph}} \cr} $$