If you were a Librarian, which criteria would you use to remove books from the shelf/catalog?

For the sake of arguement, let’s say all books are the same size. The Library you work for is at full capacity, and 10 new books come in that need to be shelved, so 10 would have to be removed. How would you do it? Would you replace like with like (Two mysteries for two mysteries)? By author? If it were me, I would first remove any duplicates. Then, any like books. My local Library has like a hundred books on Abraham Lincoln, but none on William Henry Harrison or Benjemin Harrison. Seeing there could only be so much that could be said about Lincoln, I may just go ahead and take 10 of those out. Barring that, I’d just see which 10 books have gone the longest since their last checkout and that would be the choice. Adding space or constructing a new Libary building isn’t in the budget—all the money was spent on buying 10 books. LOL

Please, this isn’t a question about First Amendment Rights or censorship or the like. Just kind of a problem solving thing. We may all have different ideas, and am curious as to other ways it could be done.