Is there a typo?
. . .Is the question "What fraction of Secondary 2 are girls?"
 The designations are far too confusing; I'll simplify the wording.
There are three classes:
The number of students in each of the three classes is the same.
The fraction of girls in is the same as the fraction of boys in
of all boys are in
What fraction of the B students are girls?
Let = number of students in each class.
"The fraction of girls in is the same as the fraction of boys in ."
. . 
The total number of boys is:
" of all boys are in "
. . . . . . . . . . . . . .
From , we have: .
Therefore, of class B are girls.