it is no.3 because the students have been determined by it's quantity while others is a not well defined set because there is no such answer because you don't know if it is rich or no.of cities and math teachers in a such particular
I think it would be numbers
13 <= x <= 30
NOTE: <= is less than or equal, just change it to
WHY 13 <= x <= 30?
x is greater than 13, but not exceeding 30