A union probability is denoted by P(X or Y), where X and Y are two events. P(X or Y) is the probability that X will occur or that Y will occur or that both X and Y will occur. The probability of a person wearing glasses or having blond hair is an example of union probability. All people wearing glasses are included in the union, along with all blondes and all blond people who wear glasses.
According to Professor Franz Kurfess of California Polytechnic State University, San Luis Obispo, union probability of two independent events A and B can be denoted as:
P(A ∪ B) = P(A) + P(B) - P(A ∩ B)
= P(A) + P(B) - P(A) * P (B)