Correct Answer - Option 3 : 27.1 L

**Given:**

Barrel of grape juice = 100 L

10 L of juice replaced with milk = 3 times

**Formula used:**

Final Q = Initial Q × (1 - (volume removed/Initial Q)^{n}

Here, n = Number of times volume taken out

**Calculation:**

Final Quantity = Initial Q × (1 - (volume removed/Initial Q)^{n}

⇒ 100 × (1 – (10/100))^{3}

⇒ 100 × (9/10) × (9/10) × (9/10)

⇒ 729/10

⇒ 72.9 L

Now, Milk added = 100 – 72.9

⇒ Milk added = 27.1

**∴**** 27.1 L of milk is to be added**