Hi. Recently I've been wondering if i could generalize the following problem I randomly had in mind: There are 15 kinds of identical books in a book store . The book store owner packs books into boxes that can contain up to 4 books . Assume that all books are of the same size and those boxes must be completely filled up , how many different ways can the boxes be packed ? Can anyone give me advice on finding out the relationship between these 2 variables ? Or is there any ?