Rearranging the definition of conditional probability, $~$\\mathbb P(X \\wedge Y) \= \\mathbb P(Y) \\cdot \\mathbb P(X|Y).$~$ E\.g\. to find "the fraction of all patients that are sick and get a positive result", we multiply "the fraction of patients that are sick" times "the probability that a sick patient blackens the tongue depressor"\.

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