Answer:

Explanation:
21 students like pizza which is the same as 3/4 of the class like pizza. The total amount of students in the class is x so 3/4 of x (or the class like pizza).
Therefore you get this equation:

Next, you divide 21 by 3/4, and 3/4x divided by 3/4 so you get..

x=21*(4/3)=28