One may obtain the possible number of combinations of a set of elements, such as the set [A, B, C, D], which has 4 elements.
In the above case, with the number of elements being 4, we can write the answer simply as 4!, expressed as “4 factorial”.
4! = 1x2x3x4 = 24
Simply multiply the numbers from 1 up to the total number of elements being considered to derive the number of combinations possible with that number.
In the example [X, Z], we have: 2!=1×2=2 X Z, Z X
For the example [X, Y, Z], we have 3!=1x2x3=6 X Y Z, X Z Y, Y Z X, Y X Z, Z X Y, Z Y X
Try figuring the possible batting order combinations for a baseball team!!!