"A ∩ B" refers to the total number of books signed by both parties;

"B ∩ C" refers to the total number of books signed by both parties;

"A ∩ B ∩ C" is the total number of books signed by Nail, B and C simultaneously.

It can be concluded that the total number of books borrowed by A, B and C is: A+B+C-(A ∩ B+A ∩ C+B ∩ C)+A ∩ B ∩ C.

Because the first six items are quantitative, the bigger the "A ∩ B ∩ C", the more books three people borrow. Because A ∩ C is 25, A ∩ B ∩ C is 25 at most. Substituting the above formula, we can see that the total number of books borrowed by A, B and C is 67, and the total number of books not borrowed by A, B and C is at least 33.