math

You want to answer:

How many total number of students are there in the school?
How many students would be covered if you only put 30 students in each of the 18 classes?
How many students would be left out?
How many classes of 30 students would be required to cover these left over students?

18 classes with 35 students comes to 630 students. In order to have a class size of 30, there would need to be 630/30 = 21 classes. Thus, since there are already 18 classes, you must add 3 more to achieve this class size.