Basic Probability
\[ P(E_1 \cap E_2) = P(E_1) \cdot P(E_2 \mid E_1), \quad \text{where } P(E_2 \mid E_1) = \frac{P(E_1 \cap E_2)}{P(E_1)} \]
\[ P(E_1 \cap E_2) = P(E_1) \cdot P(E_2 \mid E_1), \quad \text{where } P(E_2 \mid E_1) = \frac{P(E_1 \cap E_2)}{P(E_1)} \]