Answer:
30
Step-by-step explanation:
boys:  girls  : total
3 Â Â Â Â Â 7 Â Â Â 3+7 =10
The total number of students must be a multiple of 10
The class size is between 22 Â and 34
The only multiple of 10 between those numbers is  30
10,20,30,40
Therefor the class size must be 30