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How about this short version ?
After day 1, you have 30 liters of mixture, containing 25 liters of wine and 5 liters of water.
The proportion water to total is = in volumes.
When next day 12 liters of the mixture was removed, 30-12 = 18 liters of mixture remained in the container.
These 18 liters of the mixture contain volume of water, i.e. = 3 liters of water.
When 12 liters of water was added, then the total volume of water in the mixture became 3 + 12 = 15 liters.
Thus final fracture of water is = of the total volume. ANSWER
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Surely, it is the same solution as by Jim, only presented in other form.
Now, when you have one long and one short presentation, you can choose which is better for you.
My personal opinion is that IF the shorter solution does not lose nothing essential, THEN it is better.
But you may think differently . . .
Some people understand text much better, when it is shorter.
In short version, I can express, highlight and explain the main ideas MUCH BETTER than ANYONE ELSE in long version.
Regarding long versions, I am 129% sure that NOBODY even reads them,
and 179% sure that nobody understands them, even if read them.