With help of permutations and combinations we can calculate the probability of some event to happen,if the event is composed from several other events (when is a list)
If the order of the events doesn't matter then we have a combination, if the order do matter then we have a permutation. We can say that a permutation is an ordered combination.
The number of permutations of n objects taken r at a time is determined by the following formula:P(n,r)=n!(n-r)!
The number of combinations of n objects taken r at a time is determined by the following formula: C(n,r)=n!/(n-r)!r!