A sample of x litres from a container having a 60 litre mixture of milk and water containing milk an...........

Explanation:

The best way to solve this problem is to go from the answer choices.
The mixture of 60 litres has in it 24 litres of milk and 36 litres of water. (2 : 3 :: milk : water) 
When you remove x litres from it, you will remove 0.4x litres of milk and 0.6x litres of water from it.
Take choice (2). According to this choice, x=10.
So, when one removes, 10 litres of the mixture, one is removing 4 litres of milk and 6 litres of water.
Therefore, there will be 20 litres of milk and 30 litres of water in the container.
Now, when you add 10 litres of milk, you will have 30 litres of milk and 30 litres of water - i.e. milk and water are in equal proportion.

Hence, option 2 is the correct answer.