Ravi row to a place 45 km distant and back in 20 hours. He finds that he can row 12 km with stream in the same time as 4 km against the stream, then the speed of the stream is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3 km/hr
In equal times, distances are proportional to speeds. So the downstream speed is 3 times the upstream speed. Let the upstream speed be v km/hr; then the downstream speed is 3v km/hr.
For the round trip of 45 km each way:
\( \displaystyle \frac{45}{3v}+\frac{45}{v}=20 \)
\( \displaystyle \Rightarrow\ \frac{15}{v}+\frac{45}{v}=20 \)
\( \displaystyle \Rightarrow\ \frac{60}{v}=20 \)
\( \displaystyle \Rightarrow\ v=3 \)
\( \displaystyle \text{Upstream speed}=3\ \text{km/hr} \)
\( \displaystyle \text{Downstream speed}=9\ \text{km/hr} \)
The speed of the stream is half the difference of the two speeds:
\( \displaystyle \text{Speed of stream}=\frac{9-3}{2}=3\ \text{km/hr} \)