Usually (at the school math level) you are given a large finite set of events A and its subset of events B.
The events of A are (usually) supposed to be equally likely, and a typical question is
"what is the probability for the event from A to be the event from B ?"
In this situation, the probability is simply the ratio of numbers |B| to |A|
P(B) = ,
where |A| is the number of elements/events in the set A, while |B| is the number of elements/events in the subset B.
