조건부확률

${\mathrm{P}} ( A \cup B ) = {\mathrm{P}} ( A ) + {\mathrm{P}} ( B ) - {\mathrm{P}} ( A \cap B )$에서 ${\mathrm{P}} ( A \cap B ) = {\mathrm{P}} ( A ) + {\mathrm{P}} ( B ) - {\mathrm{P}} ( A \cup B )$ $\displaystyle = \frac{2}{5} + \frac{4}{5} - \frac{9}{10} = \frac{3}{10}$ 이므로 $\displaystyle \mathrm{P} ( \mathit{B} | A ) = \frac{{\mathrm{P}} ( A \cap B )}{{\mathrm{P}} ( A )}$$\displaystyle = \frac{\frac{3}{10}}{\frac{2}{5}} = \frac{3}{4}$