We are told that the ratio of male students to female students is 3:4. If there are 18 male students, then the number of female students must follow the same ratio. That is:
Where "x" is the number of female students. To determine "x" we multiply both sides by "x", we get:
Now we multiply both sides by 4/3, we get:
Solving the operations we get:
Therefore, there are 24 female students.