A man can swim at 10 km/hr in still water. It takes him twice as much time to reach the destination when he swims up the river as it takes when he swims down the river. At what speed the river is flowing?

Let the speed of the river = X km/hr

So, speed of man with the flow (downward speed) = 10 + X

Similarly speed of man against the flow (upward speed) = 10 – X

Let the man covers a distance Y.

So time taken to cover the distance Y when he swims up the river would be double of the time taken to cover the distance Y when he swims down the river.

So, ^Y⁄_10-X = 2 *^Y⁄_10+X

10Y + XY = 20Y -2XY

XY + 2XY = 20Y – 10 Y

3XY= 10Y

3X =^10Y⁄_Y

3X = 10 X =¹⁰⁄₃km/hr (Option A)