The units before and after this question are different from "1". According to a key idea in solving the application problem of fractions: grasping the same while changing, we can know that the number of girls has not changed, which is also the basis of the equation in this problem.

We can assume that the number of boys is X and the number of girls is 2/3x-2 (the class size is 5/3x-2); When the number of boys decreases by one, the class size becomes (5/3x-2- 1), and the number of girls can be expressed as 1/3(5/3x-2- 1)+3. Because the number of girls has not changed, the equation is obtained:

2/3x-2 = 1/3(5/3x-2- 1)+3

Solving this equation gives x=36.

The number of girls is 36×2/3=24 (persons).