The speed of a swimmer is 4 km h$$^{-1}$$ in still water. If the swimmer makes his strokes normal to the flow of river of width 1 km, he reaches a point 750 m down the stream on the opposite bank. The speed of the river water is ______ km h$$^{-1}$$.

The swimmer swims perpendicular to the river flow with speed $$v_s = 4$$ km/h. The river has width $$d = 1$$ km.

To begin, time to cross the river,

$$ t = \frac{d}{v_s} = \frac{1}{4} \text{ h} $$

During this time, the river carries the swimmer downstream by 750 m = 0.75 km.

Next, speed of river,

$$ v_r = \frac{\text{drift}}{t} = \frac{0.75}{1/4} = 0.75 \times 4 = 3 \text{ km h}^{-1} $$