We do rent most textbooks. Buy a book for x, use it for the semester, sell it back for y (y <= x), and the rental cost per day is (x-y)/numDays. This doesn't

We do rent most textbooks. Buy a book for x, use it for the semester, sell it back for y (y <= x), and the rental cost per day is (x-y)/numDays. This doesn't work for the new editions problem of course.