Explanation:

- To solve, we need to find the speed of the boat in still water (b) and the speed of the stream (s).

- Use the given conditions: 3 km upstream and 5 km downstream take 55 minutes, and 4 km upstream and 9 km downstream take 1 hour 25 minutes.

- Convert the time into hours: 55 minutes = 11/12 hours; 1 hour 25 minutes = 1.4167 hours.

- Set up and solve the equations:

- For the first condition: $(3/(b-s))+(5/(b+s))=11/12$

- For the second condition: $(4/(b-s))+(9/(b+s))=1.4167$

- Solve these equations to find b (speed in still water) and s (speed of the stream).

- Calculate time for 43.2 km downstream: $Time=43.2/(b+s)$

- Check options given:

- Option 1: 3.6

- Option 2: 4.4

- Option 3: 5.4

- Option 4: 4.8

- After calculations, find that the time for 43.2 km downstream aligns with option 4.

- Correct Answer: Option 4 - 4.8 hours

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