Permutations
A selection of objects in which the order of the objects matters.
nPk = n! / (n - k)!
Example: If we wanted to get the number of permutations in which 6 people could be seated into 4 chairs, we’d find that it would be 360 permutations in total:
6P4 = 6! / (6 - 4)! = 720 / 2 = 360
Combinations
The number of possible combination of r objects from a set on n objects.
nCk = n! / ( k! * (n - k)! )
Example: Let’s say instead we want to get the number of combinations in which 6 people could be seated into 4 chairs; we’d find that it would be 15 combinations in total:
6C4 = 6! / ( 4! * (6 - 4)! ) = 720 / ( 24 * 2 ) = 720 / 48 = 15