If $P(A)=0.5$ and $P(B/A)=0.4$, then by the multiplication theorem, $P(A\cap B)$ equals

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 $P(B)$.