Correct Answer - Option 3 : 15 litres

**Given:**

Total amount of mixture = 45 l

Ratio of milk and water = 4 : 1

**Calculation:**

Amount of milk = (4/5)45 = 36 l

Amount of water = 45 - 36 = 9 l

Let x litres of water to be mixed to make the milk and water ratio 3 : 2

Then, according to the question:

36/(9 + x) = 3/2

⇒ 24 = 9 + x

⇒ x = 15

**∴ 15 litres of water must be added.**