A person can row a boat in still water at the rate of 5 km/hr. It takes him 4 times as long to row upstream of a river as to row downstream to cover same distance in the same river. The speed of flow of the stream is

3 km/hr

The correct answer is Option (2) → 3 km/hr

Let speed in still water = $5$ km/hr

Let speed of stream = $u$ km/hr

Upstream speed = $5 - u$

Downstream speed = $5 + u$

Given: time upstream is 4 times time downstream

$\frac{d}{5-u} = 4\cdot \frac{d}{5+u}$

Cancel $d$:

$\frac{1}{5-u} = \frac{4}{5+u}$

$5+u = 4(5-u)$

$5+u = 20 - 4u$

$5u = 15$

$u = 3$

Final answer: $3$ km/hr