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If the set S is divided into subsets B_1, B_2 which B_1 ∪ B_2 = S, B_1 ∩ B_2 = ∅ and P(B_1) gt; 0 and P(B_2) gt; 0, then for any event A ⊂ S below formula is true. P(A) = P(A|B_1) * P(B_1) + P(A|B_2) * P(B_2). If in above way we divide the set S into mutually exclusive three subsets B_1, B_2, B_3 then we get P(A) = P(A|B_1) * P(B_1) + P(A|B_2) * P(B_2) + P(A|B_3) * P(B_3). Similar we can define the formula for any number of sets B_1, B_2, B_3,...