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Let us assume the amount of Milk in the container to be X and the amount of water in the container to be Y.
We are told that X + Y = 300
Now, an operation is given where "an amount of this solution was taken out from the container which was twice the volume of water that was earlier poured into it, and water was poured to refill the container again" Volume of the water initially is Y. If twice that amount is taken out, the percentage of the solution that is taken out will be, 
That means the quantity of milk that will remain in the solution will be, 
This value is given to be 72%, 72% of 300 will be 216

Writing X = 300 – Y
(300 – Y) (300 – 2Y) = 64800
Expanding this we have,
2Y² – 900Y + 25200 = 0
Factorising this equation we have,
2(Y – 30) (Y – 420) = 0
Y is either 30 or 420.
Given that the capacity of the container itself is 300, Y has to be 30.
Hence the amount of water initially is 30 Litres.