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.