Now girls are boys: 5 ÷6=5/6.
Boys have:
2 ÷(5/6-3/4)
=2 ÷( 10/ 12-9/ 12)
=2 ÷ 1/ 12
=24 people
There used to be 24x3/4 girls = 18.
It turns out that there are 24+ 18=42 people in the whole class.
Analysis: In this problem, the number of girls and class size are changing, but the number of boys remains the same. The key to solve this problem is to grasp the invariants in the problem. According to the first condition, we can get the score of a girl who is a boy, and then according to the third condition, we can get the score of a girl who is a boy now. The difference between these two types is the corresponding score of two girls who transferred later. Calculate the unit of the number of boys = 1 with the amount corresponding to the score, and then calculate the original number of girls. The numbers of boys and girls have been worked out. Add them together and you can work out the original number of students in this class.