Raymond has made up two vinaigrettes for his restaurant. One has half oil and half vinegar, the other has 2/3 oil and 1/3 vinegar. He has 180ml of each. He wants to use them to make 2 x 180ml, each containing the same proportions, so he removes 90ml from the one with equal proportions and adds it to the other. He mixes well and takes 90ml from the new larger mix and adds it to the small one, so he once again has 2 x 180ml.