Explanation:

To solve this problem, we need to find the speed of the stream:

- Downstream:

- Distance = 20 km

- Time = 2 hours

- Effective speed = Distance/Time = 20/2 = 10 km/h

- Upstream:

- Distance = 20 km

- Time = 5 hours

- Effective speed = Distance/Time = 20/5 = 4 km/h

- Concept:

- Let the speed of the boat in still water = B km/h

- Speed of the stream = S km/h

- Downstream speed = B + S

- Upstream speed = B - S

- Equations based on the problem:

1. B + S = 10

2. B - S = 4

- Solving these equations:

- Add both equations: 2B = 14 => B = 7

- Substitute B in either equation to find S: S = 3

- Correct Option: 1

- ?? 3 km/h is the speed of the stream.

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