Explanation:

Alright, let’s tackle your question step by step:

- The boat travels 3 km upstream and 5 km downstream in 55 minutes (which is 11/12 hours).

- It also does 4 km upstream and 9 km downstream in 1 hour 25 minutes (which is 17/12 hours).

- Let's call speed upstream u and speed downstream d.

Set up the equations:

- 3/u + 5/d = 11/12

- 4/u + 9/d = 17/12

Now, multiply first equation by 4 (to help eliminate variables):

- 12/u + 20/d = 44/12

Multiply second equation by 3:

- 12/u + 27/d = 51/12

Subtract the first modified from second:

- (12/u + 27/d) - (12/u + 20/d) = (51 - 44)/12

- 7/d = 7/12 ? d = 12 km/h

Plug d back in one equation:

- 3/u + 5/12 = 11/12

- 3/u = 6/12 = 1/2 ? u = 6 km/h

Now, for how long to go 43.2 km downstream at 12 km/h?

- Time = 43.2 / 12 = 3.6 hours

Here’s what this all means:

- Option 1 - 3.6 hours: The math checks out.

- Option 2 - 4.4 hours: Too long.

- Option 3 - 5.4 hours: Not right.

- Option 4 - 4.8 hours: Also too long.