A man rows a boat such that it covers a distance upstream in twice the time taken by him to cover the same distance downstream. If the speed of stream is 5 km/h, the speed of boat in still water is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 15 km/h
Let the speed of the boat in still water be b km/h. Then the upstream speed is \(b - 5\) and the downstream speed is \(b + 5\).
For the same distance, time is inversely proportional to speed. Upstream takes twice as long, so the downstream speed is twice the upstream speed:
\(b + 5 = 2(b - 5)\)
\(b + 5 = 2b - 10\)
\(b = 15\)
So the speed of the boat in still water is 15 km/h.