In how many ways can you put the 30 books in a row on a shelf if the novels are on the left,the math books are in the middle and the history books are on the right?
(The numbers of books of each subject are not known)
In the limit, i.e. if all 30 are e.g. novels, there are 30! ways.
If there are 10 of each, then it's 3 x 10!.
So suppose there are x novels, y math books and z history books.
Then there are arrangements.
It can be simplified by letting : there are arrangements.
OTOH the answer could well be "Only one." If you're a manic arranger so your books are arranged neatly like that, you're going to arrange them in a rigid order within the categories.