여사건 합사건 확률

$\displaystyle {\mathrm{P}} ( A ) = \frac{1}{2}$, $\displaystyle {\mathrm{P}} \left( A \cap B ^{C} \right) = {\mathrm{P}} ( A ) - {\mathrm{P}} ( A \cap B ) = \frac{1}{5}$에서 $\displaystyle {\mathrm{P}} ( A \cap B ) = \frac{3}{10}$ $\displaystyle {\mathrm{P}} \left( A ^{C} \cup B ^{C} \right) = {\mathrm{P}} \left( ( A \cap B ) ^{C} \right) = 1 - {\mathrm{P}} ( A \cap B ) = 1 - \frac{3}{10} = \frac{7}{10}$