S - the set of all equally likely outcomes, n(S) - total number of outcomes in S, A - the event, n(A) - total number of outcomes in A. The probability of A happening is given by formula P(A) = n(A) / n(S). Example. Rolling the die. What is the probability of getting an even number? S = {1, 2, 3, 4, 5, 6} n(S) = 6. A = {2, 4, 6}. n(A) = 3. P(A) = n(A) / n(S) = 3 / 6 = 1 / 2.