A joke that played tricks on my uncle

David likes to play, and his exam results are always poor. His father warned him that if he went on like this, he would not be allowed to go to Italy for a holiday with his family.

David said to the teacher after the exam, would you please give me 100 this time?

The teacher said: how can this be done? Can only get 20 points.

David thought for a moment and said, well, give 100 this time, and then deduct 10 every time until 80 points are deducted.