A person can row a motor boat in still water at the rate of 12 km/hr. It takes him thrice as long to row upstream of a river as to row downstream of the same river. The speed of the stream is :
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 6 km/hr
Let the speed of the stream be s km/hr. Then
\( \displaystyle \text{Upstream speed}=12-s,\qquad \text{Downstream speed}=12+s \)
For the same distance, time is inversely proportional to speed. Upstream takes three times as long, so the downstream speed is three times the upstream speed:
\( \displaystyle 12+s=3(12-s) \)
\( \displaystyle 4s=24 \)
\( \displaystyle s=6\text{ km/hr} \)
CBSE Applied Mathematics (Class XII):
Boats and Streams
