If $P(A)=0.5$ and $P(B/A)=0.4$, then by the multiplication theorem, $P(A\cap B)$ equals
A$0.9$
B$0.2$
C$0.1$
D$0.4$
Answer & Solution
Correct answer: B. $0.2$
Using the multiplication theorem, $P(A\cap B)=P(A)\,P(B/A)=0.5\times 0.4=0.2$. A common error is to add the probabilities or to confuse $P(B/A)$ with $P(B)$.
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