Very challenging = Intersection in many dimensions
A man reads a book, which has x% of all knowledge in psychology. He reads a second book, which has also x% of all knowledge in psychology. But as he can't get more than 100% of all knowledge in the topic, the second book has intersections in knowledge with the first one (a fraction of their content is similar). The intersection is a%. All the other books in the individual's collection intersect themselves in this fraction of a%. If the man has read (n-1) books, how much more knowledge will the Nth book provide?