### 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.

A. 60 km/hr

B. 30.5 km/hr

C. 57 km/hr

D. 67.2 km/hr

$\begin{array}{l}\text{Average speed}\\ =\frac{2xy}{x+y}\mathrm{~km}/\mathrm{hr}\\ =\frac{2\times84\times56}{84+56}\mathrm{~km}/\mathrm{hr}\\ =\frac{2\times84\times56}{140}\mathrm{~km}/\mathrm{hr}\\ =67.2\mathrm{~km}/\mathrm{hr}\end{array}$