There are forty students in my class, of who about a third are female.
[There are forty students in my class "and" about a third "of them" are female.]를 바꾼 문장으로, Of them(혹은 of forty students:목적격)을 바꾼 절이니까 목적격 whom이 들어가서 There are forty studenta in my class of whom a third are female. 인 거 같습니다.