Suppose we lay down two cups in front of you. One of the cups is filled with tea and the other one with coffee. Now we ask you to take a spoonful of tea and mix it with the coffee. At this moment, the coffee cup has a mixture of tea and coffee. You have to take that mixture (spoonful) and add it back to the tea.
Can you now tell if the cup of coffee has more tea or the cup of tea has more coffee?
Both will have the same amount.
Let’s say there’s 100ml of tea and 100ml of coffee initially and the teaspoon holds 10ml of liquid. After transferring 10ml of liquid from the tea to the coffee cup and another 10ml from the coffee to the tea cup, the quantity will remain the same – 100ml of coffee and tea mix in the tea cup and 100ml of coffee and tea mix in the coffee cup.
So let’s say after the mixing is done there is 99ml of tea and 1ml of coffee in the tea cup. The 1ml of tea has to be in the coffee cup. That means that the coffee cup has 99ml of coffee and 1ml of tea.
In other words, if there’s (X)ml of tea missing from the tea cup, then there has to be an equal amount of of coffee missing from the coffee cup.