In general, the information of known topics will be listed by formulas. Note that the question is about the proportion of people who didn't participate in class A and class B, so we try to change all the known situations into words that A and B didn't participate.

It is known that the number of class A participants is exactly 1/3 of the number of class B participants, and the number of class B participants is exactly 1/4 of the number of class A participants ... and because the number of students in the two classes is equal.

So what we can know is that 1/3 B class and A class are equal to 1/4 A class and B class. Later, after a simple calculation, the proportion of class A and class B who did not participate was 8/9.

I hope I can help you. Let me start with a topic. Here comes the leader. I'm still at work. If it helps you, I'll talk about other topics when I get home from work.