4 books of mathematics ,6 books of physics and 2 books of chemistry are to be put on a shelf . How many arrangments are there if the books of the same subject have to be grouped together ???
3! (4! X 6! X 2!)
There are n! ways of arranging n objects. So we get, for example, 6! ways of organising the physics books. But we multiply but 3! because there are 3 catagories of books and so 3! ways of ordering these catagories.