Suppose you have 30 books (15 novels, 10 history books, and 5 math books). Assume that all 30 books are different.
In how many ways can you put the 30 books in a row on a shelf?
30! (where n! = n*(n-1)*(n-2)*...*3*2*1)
In how many ways can you get a bunch of four books to give to a friend?
C(30,4) where C(n,r) = n!/((n-r)!r!)
In how many ways can you get a bunch of three history books and seven novels to give to a friend?
C(10,3)*C(15,7)
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?
15!*5!*10!
In how many ways can you put the 30 books in a row on a shelf if the five math books are to be grouped together, but there are no restrictions on the placement of the other books?
Treat the 5 math books as a single unit, temporarily. This one unit, combined with the remaining 25 books, can be arranged in
(25+1)! = 26! ways
But for each one of these arrangements, the 5 math books can be arranged in 5! ways, therefore the total is
26!*5!