Question: You have a cup of tea (A) and a cup of coffee (B) (equal amounts), and you take a spoonful of the coffee (B) and mix it thoroughly into the tea (A). Then you take a spoonful of tea (A) and mix it with the coffee (B). Does the cup that originally had tea in it (A) have more coffee or does the cup that originally had coffee in it (B) have more tea?
Answer: Both cups end up having the same amount of the other liquid in it. When you take the spoonful of coffee (B) and put it in the tea (A) it increases the volume of the tea (A) by that spoonful. Then when you take a spoonful of the tea (A), part of that spoonful is coffee, taking away the proportional amount of coffee. This makes the amount of coffee in the tea and tea in the coffee equal.