The speed of a swimmer is 4   km   h - 1 in still water. If the swimmer makes his strokes normal…

Let $v_r$ be velocity of river and $v_m$ be velocity of man.

Time taken by man to cross the river of width $=1000 \mathrm{m}$ is $t=\frac{1}{4} \mathrm{h}$.

Now, drift $x=v_r\times t$

$$\begin{gathered} \Rightarrow 0.750=v_r\times \frac{1}{4} \\ \Rightarrow v_r=3 \mathrm{km} {\mathrm{h}}^{-1} \end{gathered}$$