Explanation:

Let’s break it down:

- The boat’s speed in still water: 15 km/h.

- Let’s call the stream speed x km/h.

- Downstream speed: 15 + x

- Upstream speed: 15 - x

- Downstream distance: 30 km

- Upstream distance: 30 km

- Total time: 4.5 hours

Now the time equation:

Time = Distance/Speed.

So: (30?/?(15+x)) + (30?/?(15–x)) = 4.5

Let’s solve:

(30/(15+x)) + (30/(15-x)) = 4.5

Multiply both sides by (15+x)(15-x):

30(15-x) + 30(15+x) = 4.5(15^2 - x^2)

30*15 - 30x + 30*15 + 30x = 4.5(225 - x^2)

450 + 450 = 4.5(225 - x^2)

900 = 1012.5 - 4.5x^2

4.5x^2 = 1012.5 - 900 = 112.5

x^2 = 25

x = 5

So here’s the answer:

- Option 1: 4 – nope, too low.

- Option 2: 7 – too high.

- Option 3: 5 – This is correct.

- Option 4: 3 – too low.

Correct Answer: Option 3 – 5 km/h.