Correct Answer - Option 4 : 12 lt.

**Given:**

Quantity of the mixture = 45 litre

The ratio of the milk and the water = 3 ∶ 2

**Calculation:**

The quantity of milk in the mixture = (3/5) × 45 = 27 litre

The quantity of the water in the mixture = (2/5) × 45 = 18 litre

Let the quantity of water added to make the ratio 9 ∶ 10 be x

Now, A/Q

27/(18 + x) = 9/10

⇒ 27 × 10 = 9(18 + x)

⇒ 3 × 10 = 18 + x

⇒ 30 – 18 = x

⇒ x = 12

**∴ The quantity of water to be added is 12 litre**