Factorials etc.

x objects in every possible order = x!

e.g. 5 objects = 5 x 4 x 3 x 2 x 1 = 5! = 120 possible combinations.

x = no. of variables
y = no./group

       x!      
 y! (x - y)!

= number of combinations where each variable only appears once per group.

 (x + y - 1)! 
   (x- 1)! y!

= number of combinations where variables can appear more than once in each group.