The conditional probability formula states that given events A and B,
[tex]P(A|B)=\frac{P(A\cap B)}{P(B)}[/tex]Therefore, in our case,
[tex]\begin{gathered} P(B|A)=\frac{P(A\cap B)}{P(A)} \\ \Rightarrow P(A\cap B)=P(B|A)*P(A)=\frac{63}{100}*\frac{1}{2}=\frac{63}{200} \end{gathered}[/tex]