A swimmer moving upstream passes a float at point P. After two hour, he returns and at the instant swimmer meets the float again is at a distance 15 km from the point P. If the swimmer velocity in water remains constant, then find the speed of the river flow.

In the frame of river, the float remains stationary. The speed of swimmer with respect to river is $v_{rel}.$
Thus, the time taken by swimmer to return the float position is $t_0=2+2=4h$
In this time, the float moves 15 km in the frame of ground.
$\therefore$ The river flow velocity = velocity of float in the frame of ground $=\frac{15}{4}=3.75km{h}^{-1}$