Suppose that {B1, B2} is a partition of a sample space such that P(Bi) > 0,

Suppose that {B1, B2} is a partition of a sample space Ω such that P(Bi) > 0, for i = 1, 2. Then, for any event A in Ω, we have P(A) = P(A|B′

1)P(B′

1) + P(A|B′

2)P(B′

2).