Answer:
6 students.
Step-by-step explanation:
3v + Â c = 22 Â ---> c = 22 - 3v
2v + 4c = 28 ---> 2v + 4(22 - 3v) = 28
             2v + 88 - 12v = 28
            -10v + 88 = 28
                - 88  - 88
            - 10v = - 60
               v = 6 students