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1. 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

Solution:

\begin{aligned} Average speed \\ = \frac{2 x y}{x + y} km/hr \\ = \frac{2 \times 84 \times 56}{84 + 56} km/hr \\ = \frac{2 \times 84 \times 56}{140} km/hr \\ = 67.2 km/hr \end{aligned}

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