Anna’s sister, Claire, has \(10\) novels, \(11\) textbooks and \(12\) comic books on her shelf. She also doesn’t like to keep all her books there. Each time she takes two books of different type from the shelf and puts a book of the third type back on. So for example, she might take a novel and a comic book and put a textbook back. Show that eventually there is only a single textbook, and exactly a textbook, left on her shelf.