Correct Answer - Option 2 : 10 liters

**Given:**

Quantity of milk in the tub = 30 liters

The cost price of milk = 20/liter

The cost price of new mixture = 15/liter

**Concept used:**

The new price of milk = (Total cost price of the milk)/(New quantity of the mixture)

**Calculation:**

The total cost price of the milk = 20 × 30

⇒ 600

Let the quantity of water added to the tub be x liters.

Then, 600/(30 + x) = 15

⇒ 600 = 450 + 15x

⇒ 15x = 150

⇒ x = 10

**∴**** The quantity of water added in the tub is 10 liters.**