In an experiment, the probability of an event is the likelihood of that event occuring.
Probability is a value between (and including) zero and one.
If P(E) represents the probability of an event E, then:
A listing of all of the outcomes of an experiment is called the sample space (S) of the experiment and n(S) represents the number of outcomes in the sample space.
If n(E) represents the number of outcomes in event E, then: