A man rows 15 km upstream in 5 hours and 25 km downstream in 5 hours each time, then the speed of the stream is

1 km / hour

The correct answer is Option (4) → 1 km / hour **

Upstream speed = $\frac{15}{5} = 3$ km/hr

Downstream speed = $\frac{25}{5} = 5$ km/hr

Let boat speed in still water = $b$, stream speed = $s$.

Upstream: $b - s = 3$

Downstream: $b + s = 5$

Add both:

$2b = 8 \Rightarrow b = 4$

Substitute:

$4 - s = 3 \Rightarrow s = 1$

Speed of stream = 1 km/hr