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 the speed of the stream be u km/hr. Then the upstream speed is \(5 - u\) and the downstream speed is \(5 + u\). For the same distance d, the upstream time is 4 times the downstream time:
\( \displaystyle \frac{d}{5 - u} = 4 \times \frac{d}{5 + u} \)
\(5 + u = 4(5 - u)\)
\(5 + u = 20 - 4u\)
\(5u = 15,\quad u = 3\)
So the stream flows at 3 km/hr. Check: upstream 2 km/hr, downstream 8 km/hr, a ratio of 4.
CBSE Applied Mathematics (Class XII):
Boats and Streams
