교사건의 확률

[출제의도] 확률의 덧셈정리 이해하기 ${\mathrm{P}} \mathit{(} A \cup B )$$= {\mathrm{P}} \mathit{(} A ) + {\mathrm{P}} \mathit{(} B ) - {\mathrm{P}} \mathit{(} A \cap B )$ $= 4 \times {\mathrm{P}} \mathit{(} A \cap B ) - {\mathrm{P}} \mathit{(} A \cap B )$ $= 3 \times {\mathrm{P}} \mathit{(} A \cap B )$ $\displaystyle \frac{2}{3} = 3 \times {\mathrm{P}} \mathit{(} A \cap B )$에서 $\displaystyle {\mathrm{P}} \mathit{(} A \cap B ) = \frac{2}{9}$