Explanation:

To find the speed of the boat in still water, we need to use the concept of relative speed in upstream and downstream situations. Let's break it down:

- Upstream:

- Distance = 18 km

- Time = 3 hours

- Speed = Distance/Time = 18 km ÷ 3 hours = 6 km/h

- This speed is equal to (Speed in still water - Speed of stream)

- Downstream:

- Distance = 40 km

- Time = 4 hours

- Speed = Distance/Time = 40 km ÷ 4 hours = 10 km/h

- This speed is equal to (Speed in still water + Speed of stream)

- Formulas:

- Let the speed of the boat in still water = \( b \) km/h

- Let the speed of the stream = \( s \) km/h

- From upstream, \( b - s = 6 \)

- From downstream, \( b + s = 10 \)

- Solving equations:

- Add both equations: \( (b - s) + (b + s) = 6 + 10 \)

- \( 2b = 16 \)

- \( b = 8 \) km/h

- Options Review:

- Option: 2 - 8 km/h is the correct answer.