First notice that at the end of the process, there is one liter of fluid in the wine jug and one liter of fluid in the water jug. Let W be the milliliters of wine in the water jug. Then there must be 1000 - W milliliters of water in this jug. That leaves W milliliters of water, which must all be in the wine jug! So there is exactly as much water in the wine jug as there is wine in the water jug.