To solve this problem, first we need to figure out what fraction of the class are boys.
If we know that 4/7 of the students are girls, there are only 3/7 students left in the class, therefore they must be boys.
We know that there are 9 boys in the class. (Let the variable x represent how many students total there are in the class.)
3/7x=9
If we divide both sides by 3/7, we get
x=21.
Therefore, there are 21 students in the class. Your answer is C. 21.
