Correct Answer - Option 1 : 10 liters

**Given:**

The total quantity of mixture = 90 litres

The ratio of milk to water is 5 ∶ 4

**Calculation:**

In 90 litres

Quantity of milk = 90 × (5/9) = 50 liters

Quantity of water = 90 × (4/9) = 40 liters

Let x liters of water is added in the mixture to make the ratio of milk to water 1 ∶ 1

⇒ (50/(40 + x)) = 1/1

⇒ 50 = 40 + x

⇒ x = 50 - 40 = 10

**∴ 10 liters of water should be added in the mixture.**