20 L of pure water was added to a vessel containing 80 L of pure milk. 36 L of the resultant mixture was then sold and some more pure milk and pure water was added to the vessel in the respective ratio of 7 : 2. If the final quantity of water was 3 L less than the initial quantity of water in the vessel, what was the quantity of pure milk that was added to the vessel? (in L)

After adding 20 L of water to 80 L of milk, the 100 L mixture has milk and water in an 80:20 ratio. Selling 36 L leaves 51.2 L of milk and 12.8 L of water, and we add 7x and 2x of pure milk and water respectively to make the new water amount 17 L. Solving 12.8 + 2x = 17 gives x = 2.1, meaning the quantity of pure milk added was 7x, or 14.7 L.

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