We need to represent the number of students in the whole class, so we will call the total number of students in the whole class N.
The boys represent 30% of N, and there are 15 boys.
So first change the 30% to 0.30, then we have
0.30 * N = 15 boys (30% of the total kids in class = 15 boys)
So divide both sides by 0.30 and find that N = 50 (there are 50 students in the whole class)
If a student is not a boy, the student is a girl, so 50 - 15 = 35 girls.