Question 185 of 308

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

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Answer: 3

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 $x={v}_{r}\times t$
$\Rightarrow 0.750={v}_{r}\times \dfrac{1}{4} \Rightarrow {v}_{r}=3km{h}^{-1}$