The ratio of milk to water in a mixture is 10:3. 39 litres of mixture is removed and 6 litres of water is added. In the final mixture, milk is double the water. Find the quantity of water in the initial mixture.

Let the quantity of the milk in the initial mixture = 10x litres
The quantity of the water in the initial mixture = 3x litres

The quantity of the milk in 39 litres of mixture = (39 / 13) × 10 = 30 litres
The quantity of the water in 39 litres of mixture = (39 / 13) × 3 = 9 litres

According to the question,
(10x – 30) = 2 × (3x – 9 + 6)
10x – 30 = 2 × (3x – 3)
10x – 30 = 6x – 6

4x = 24
x = 6

The quantity of the water in the initial mixture = 3 × 6 = 18 litres