A school has 18 classes with 35 students in each class. in order to reduce class size to 30, how many new classes must be formed?

18*35 - number of the students at school.

x - is a number of classes if it is going to be 30 students per class, then

30x - number of the students at school.

So,

18*35 = 30x

x = (18*35) / 30 = 21 classes should be if it is going to be 30 students per class.

21 - 18 = 3 new classes must be formed.