There are 30 students in a class whose average age is 20 years. The average age of boys in a class is 3 less than average age of girls in it. If the age of their class teacher, which is between 49 to 54 years is included with boys, the average of boys becomes equal to that of girls. If average age of boys as well as girls are integers, then how many boys are there in class?
1
5
2
7
3
9
4
10
5
12