
Defining the p.m.f. of X
Suppose that a school has 20 classes: 16 with 25 students in each, three with 100 students in each, and one with 300 students for a total of 1000 students.
Select a student randomly out of the 1000 students. Let the random variable X equal the size of the class to which this student belongs and define the p.m.f. of X.
Need help trying to start this problem. Any help will be greatly appreciated.

P(X=25)=.4
P(X=100)=.3
P(X=300)=.3
Since there are 400 of the 1000 kids in classes of size 25
and
there are 300 of the 1000 kids in classes of size 100
and there are 300 kids in a class of size 300.