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Answer
Assume that initially the quantity of water is xL, which gives us the quantity of milk as "80-x"L. After taking out the mentioned qty of milk and water out, we are left with 0.7xL of water and 0.3*(80-x)L of milk.
Also it is mentioned that the vessel was vacated by 55%, which means the final solution is of 0.45*80L = 36L, which can also be obtained by adding the final quantities of milk and water.
Therefore, 0.7x+0.3(80-x) = 36 gives us the value of x=30L.
