You have two 1 liter mugs – one of them halfway filled with water and the other one halfway filled with wine. You pour 300ml water from the first mug into the second one, stir it well, then pour 300ml of the mix from the second mug back to the first one. Now
If you have X ml water in the first mug, then in the second mug you have 500-X ml water and X ml wine. Therefore you have exactly as much water in the first mug as you have wine in the second one.