In order to celebrate the National Day, the experimental primary school held a prize-winning knowledge quiz in the fifth grade, with a total of 20 questions. 5 points for doing it right, 0 points for

In order to celebrate the National Day, the experimental primary school held a prize-winning knowledge quiz in the fifth grade, with a total of 20 questions. 5 points for doing it right, 0 points for not doing it and 0 points for doing it wrong. Solution: If Ya Ya did wrong X, then she did right (20-x).

5*(20-x)-2x=72

100-5x-2x=72

100-7x=72

7x=28

x=4

A: Yaya made four mistakes.