조건부확률

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