Explanation:

- You are given two scenarios involving the boat's travel downstream and upstream.

- In the first scenario, the boat takes 7.5 hours to travel 40 km downstream and 25 km upstream.

- In the second scenario, it takes 10 hours to travel 48 km downstream and 36 km upstream.

- Let $x$ be the speed of the boat in still water, and $y$ be the speed of the stream.

- The equations formed are:

- $\frac{40}{x+y}+\frac{25}{x-y}=7.5$

- $\frac{48}{x+y}+\frac{36}{x-y}=10$

- Solving these equations together helps find $x$.

Answer: Option 2, 12 km/h