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:

15 km/h

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.