Here, the factorials you mention don't have anything to do with probability - rather, they have to do with combinatorics. This is because in this context, you are talking about uniformly random elements of some set - and so counting up the elements with a specific property is a necessary step of the process.
Factorials are important because is the number of ways to list - in order - a set of
objects that are distinguishable. Because of this, it also comes up in other arrangements - such as the number of ways to choose
elements from a set of
(in an order or otherwise). Further, many other combinatorial constructions can be carried out starting with these basic ideas of arrangements, and so they involve factorials as well.
You are absolutely right that distinguishable is a keyword that screams factorial; however, distinguishability of the objects is often implicit in the problem, rather than explicitly stated. The big thing: look for any situation in which you are arranging objects in some way.
Answer from Nick Peterson on Stack ExchangeVideos
I saw this famous fact in some thead on reddit that there are less visible stars than there are possible combinations of outcomes when shuffling a deck of 52 cards.
That is by using factorial. And I've been taught that x! or "factorial" is an arithmetic process used only when elements of the group can repeat themselves, i.e. your outcome could be a deck full of aces. But this outcome is impossible.
If this is wrong, does this mean that there is a different proces than factorial that gives you even larger number?
Is there are general rule or is there a way I can tell a problem needs to be either solved by using factorials (multiplication) or exponents?