You use either
obj->AdultBook::getPurchasePrice();
or
obj->ChildrenBook::getPurchasePrice();
or
obj->Book::getPurchasePrice();
For obj
of type ComicBook
EDIT FOR Emilio Garavaglia
Lets assume that you have not redefined getPurchasePrice
for adult and childrens book, you could have this
Key A - Adult book, C - Childrens book, CB - Comic Book, B - Book