It turns out that everyone can borrow six books, and every girl can borrow nine.

(9-6)=3

When it was only for lesbians, each of them got three more books than six. When distributing it to everyone, boys only need to take away three extra books from each female classmate, and because each girl has six books, the extra books for every two girls can be given to one boy.

In other words, the number of boys is less than that of girls, which is half of the number of girls.

6 divided by 3=2

Then when all the books are distributed to boys, each boy can get his own books from two other girls (2 female pairs 1 male) besides his own six.

therefore

6+6×2= 18 copies

Well, I think we can do it without equations.

I hope I can help you.