Correct Answer - Option 3 : 5 litres

**Given**

Quantity of mixture = 40 litres

Water in mixture = 10%

**Calculation**

Quantity of Water in mixture = 10% 0f 40 = 4 litres

Quantity of Milk in mixture = 40 - 4 = 36 litres

There is 20% water in new mixture

∴ Ratio of water and milk is 20 : 80

⇒ 1 : 4

Let the quantity of water and milk in the new mixture be x and 4x

Water is added to make the new mixture, so the quantity of milk is constant throughout

⇒ 4x = 36

⇒ x = 9

So, Quantity of water in the new mixture is 9 litres

Quantity of water added = 9 - 4 = 5 litres