What is the principle of this math problem?!

It is like this:

This old man did it on purpose from the beginning, because one-half plus one-fourth plus one-sixth equals 11/12, which is basically Not enough to complete the whole. In other words, it can be understood that the old man used the remaining one-twelfth as a hidden condition, that is, property that does not belong to any of the three brothers. But this fictitious sheep did not exist. So the "I" in the title gave them a sheep in order to make them into a whole, that is, twelve sheep. The distribution based on twelve sheep can meet the old man's conditions and the hidden condition of one-twelfth, that is, this imaginary sheep will be returned to "me". This is a virtual hypothesis. In order to make the question complete without affecting the conditions of the original question, it is just a way of thinking to assist in solving the question.

Enter Liuying Ginseng.