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 speed of boat in still water = $b$ km/h.
Upstream speed = $b - 5$
Downstream speed = $b + 5$
Given: Time upstream = 2 × Time downstream
$\frac{D}{b-5} = 2\cdot\frac{D}{b+5}$
Cancel $D$:
$\frac{1}{b-5} = \frac{2}{b+5}$
Cross-multiply:
$b + 5 = 2b - 10$
$15 = b$
The speed of the boat in still water is 15 km/h.