These are all third revised editions, corresponding to the latest new curriculum. This is a set of three books from the University of Mathematics' One-to-One Correspondence Exercise series. One-to-One Correspondence Exercise A One-to-One Correspondence Exercise B One-to-One Correspondence Exercise II As shown in the photos, there are some written markings on the problem numbers, but they are still usable. Please refrain from purchasing if you are concerned about the condition, as these are used items.