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
Answer & explanation
Correct answer: option 2
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