A set of events (E1, E2, ... En) be a partition of the Sample space S, and suppose that each of them has non-zero probability of occurrence, Let A be any even associated
$$ p(A) = p(E_1) P(A | E_1) + P(E_2) . P(A | E_2) + ... + P(E_n) . P(A | E_n) \ p(A) = \Sigma^n_{j = 1} P(E_j).P(A | E_j) \
$$