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.

$$\eqalign{ & {\text{Average speed}} \cr & = {\frac{{2xy}}{{ {x + y} }}} {\kern 1pt} {\kern 1pt} {\text{km/hr}} \cr & = {\frac{{ {2 \times 84 \times 56} }}{{ {84 + 56} }}} {\kern 1pt} {\kern 1pt} {\text{km/hr}} \cr & = {\frac{{ {2 \times 84 \times 56} }}{{140}}} {\kern 1pt} {\kern 1pt} {\text{km/hr}} \cr & = 67.2{\kern 1pt} {\kern 1pt} {\text{km/hr}} \cr} $$