Factorial Notation  is used when you need to work with a series of factors, each being one less than the previous factor. 

Instead of writing out   5 · 4  · 3  · 2  · 1,
we use factorial notation and write 
5! = 120.
  
 (Note:  0! = 1  and  1! = 1)

"Piece of Cake!"

a)     6! =  6 ·5 ·4 ·3 ·2 ·1 =  720

b)     (10 - 6)!  =  4!   =   4 ·3 ·2 ·1   =  24

c)       10!       =  10 ·9 ·8 ·7 ·6 ·5 ·4 ·3 ·2 ·1
         3! · 2!          3 ·2 ·1    ·   2 ·1

                      =    3628800   =    302,400
                                 12