A swimmer can swim in still water with a speed of 5 ms-1. While crossing a river his average speed is 3 ms-1. If he crosses the river taking shortest possible path, what is the speed of flow of water ? (in ms-1)

In order to cross the river taking shortest path, the resultant velocity v of the swimmer must be perpendicular to the velocity vw of water, as shown in figure. It follows from the figure that vs2=v2+vw2 or vw2=vs2-v2=25-9=16 or vw=4 ms-1.