X+3/7X=2(X-30)
If the equation is not listed, it can be calculated as follows:
Answer: The number of students who participated is 3/7 of those who didn't.
The ratio of students who participate and those who don't is 3: 7.
∴ 3/ 10 of the total number of students participated.
Those who did not attend accounted for 7/ 10 of the total number.
When another 30 people attend, the number of students who attend is equal to the number of students who don't.
∴ When another 30 people joined, the students who participated and did not participate accounted for 5/ 10 of the total number respectively.
Later, ∴ 30 people attended, accounting for 7/10/510 = 2/10 (or 5/ 10-3/ 10).
The total number of people is 30(2/ 10)= 150.