Dividing the number of successful outcomes by the number of possible outcomes is the definition of the probability of the given event.
P(E)=n(E)/n(S)
example. let the Experiment be throwing a dice. The sample space, all the possible outcomes, is the set S={1, 2, 3, 4, 5, 6 } and n(S)=6
Let the even E be: getting an even number. The successful outcomes set is E {2, 4, 6 } and n(E)=3
So probability of getting an even number is P(E)=n(E)/n(S)=3/6=0.5
Answer: Probability of the given Event