There are 5 novels and 4 biographies. In how many ways can 4 novels and 2 biographies can be arranged on a shelf ?

4 novels can be selected out of 5 in  $5C4$ ways.
2 biographies can be selected out of 4 in  $4C2$ ways.
Number of ways of arranging novels and biographies = $5C4*4C2$  = 30
After selecting any 6 books (4 novels and 2 biographies) in one of the 30 ways, they can be arranged on the shelf in 6! = 720 ways.
By the Counting Principle, the total number of arrangements = 30 x 720 = 21600