75% is equivalent to 3/4.
Thus, since there are a total of 18 boys in the class, 18 is 3/4 of the total number of students in the class.
4/4 = total number of students.
18 x 4/3 = 24
[tex] \frac{18}{ \frac{3}{4} } [/tex] = 24
24 students in Ms. Perron's class.