배반사건의 확률

두 사건 $A$, $B$가 서로 배반사건이므로 ${\mathrm{P}} \left( A \cap B \right) = 0$ ${\mathrm{P}} \left( A \cup B \right)$$= {\mathrm{P}} \left( A \right) + {\mathrm{P}} \left( B \right) - {\mathrm{P}} \left( A \cap B \right)$$= {\mathrm{P}} \left( A \right) + {\mathrm{P}} \left( B \right)$ $\displaystyle \frac{9}{10} = \frac{2}{5} + {\mathrm{P}} \left( B \right)$ 따라서 $\displaystyle {\mathrm{P}} \left( B \right) = \frac{1}{2}$