Correct Answer - Option 3 : 65 liters

**Given:**

The initial ratio of milk to water is = 5 : 4

**Calculation:**

Let the quantity of milk and water be 5x and 4x respectively.

Initial concentration the of mixture = 5x + 4x = 9x

The initial quantity of milk = 5x

The initial quantity of water = 4x

According to the question,

[5x - {20 × (5/9)}]/[4x - {20 × (4/9)} + 10] = 5/6

⇒ (45x - 100)/(36x + 10) = 5/6

⇒ 270x - 600 = 180x + 50

⇒ 90x = 650

⇒ x = 650/90

x = 65/9

The initial concentration of the mixture = 9 × (65/9)

⇒ The initial concentration of the mixture = 65 liters

**∴** **The initial concentration of the mixture is 65 liters.**