This kind of topic just confuses the reader and cannot be thought according to the calculation of the topic.

In fact, the mother-in-law who sells melons gets 50 cents, the grandson gets 4 points, and the buyer pays 50 cents and 4 points. No problem.

It can be said that the final ownership of 60 cents is: 6 cents for buying melons (60 cents at first, 54 cents actually paid), 4 cents for grandchildren and 5 cents for selling melons.

As long as the thinking is clear, this kind of topic is not difficult.