Explanation:

- The initial mixture contains 40 litres with 25% alcohol.

- This means there are 10 litres of alcohol (25% of 40 litres).

- 10 litres of water are added, making the total volume 50 litres.

- The amount of alcohol stays at 10 litres.

- To find the new alcohol percentage: (10 litres of alcohol / 50 litres of total mixture) * 100 = 20%.

- Option 1: 27% - This is not correct as the alcohol percentage is lower.

- Option 2: 18% - This is incorrect based on calculations.

- Option 3: 20% - This is the correct alcohol percentage.

- Option 4: 25% - Incorrect as the percentage decreases with more water.