Condition on the correct denominator

P(A|B)=P(A∩B)/P(B), provided P(B)>0. Restrict the sample space to B before counting favourable cases. The multiplication rule is P(A∩B)=P(B)P(A|B)=P(A)P(B|A). In a two-way table, use the relevant row or column total for a conditional probability, and the grand total for an unconditional joint probability.