Explanation:

- Original Mixture: It contains 189 litres with an alcohol to water ratio of 5:4.

- Alcohol in Original Mixture: The fraction of alcohol is 5/(5+4) = 5/9.

- Actual Alcohol Content: 189 * (5/9) = 105 litres.

- Water Content: 189 - 105 = 84 litres.

- New Ratio Required: Alcohol to water should be 5:3.

- Equation to Solve: Let x be the additional pure alcohol. Hence, (105 + x)/84 = 5/3.

- Solving: Cross-multiply and solve (105 + x) = (5/3) * 84.

- Calculate x: (5/3) * 84 = 140; therefore, 105 + x = 140, so x = 35 litres.

Option:2, 35 litres is the correct answer.