Discussion
A group of men can row a canoe at 40 kilometers per hour in still water. After rowing for 2 hours the stream flows downhill and they take two more hours to reach a point. On the way back by the same route the time taken is two times the time taken for onward journey. What is the speed of the stream?
Answer: B
The speed of the stream is 20 km/hour.
Onward: 2 hours in still water at 40 km/h covers 80 km, then 2 hours downstream at \(40 + s\) covers \(2(40 + s)\) km, a total of 4 hours. The return takes twice as long, 8 hours: the still stretch again takes 2 hours, so the upstream stretch takes 6 hours at \(40 - s\).
\[ \begin{aligned} \frac{2(40 + s)}{40 - s} &= 6 \\ 80 + 2s &= 240 - 6s \\ 8s &= 160 \\ s &= 20 \end{aligned} \]
Check: downstream at 60 km/h for 2 hours covers 120 km, and upstream at 20 km/h the same 120 km takes 6 hours. With 40 km/h the boat could not move upstream at all, which rules out that option.
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