The speed of a swimmer is 4km h ^ - 1 in still water. If the...

Let ${v}_{r}$ be velocity of river and ${v}_{m}$ be velocity of man.

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

Now, drift $x = {v}_{r} \times t$

$⇒ 0 . 750 = {v}_{r} \times \dfrac{1}{4} ⇒ {v}_{r} = 3km{h}^{- 1}$