1. The basic probability formula is given by: $$P(E) = \frac{\text{Number of favorable outcomes}}{\text{Total number of possible outcomes}}$$ This formula finds the likelihood of an event $E$ occurring. 2. If the outcomes are equally likely, this formula applies directly. 3. For the complement of an event $E$, the probability is: $$P(E^c) = 1 - P(E)$$ where $E^c$ is the event that $E$ does not occur. 4. For the union of two events $A$ and $B$, the probability formula is: $$P(A \cup B) = P(A) + P(B) - P(A \cap B)$$ where $A \cap B$ is the event both $A$ and $B$ occur. 5. For two independent events $A$ and $B$, the probability of both occurring is: $$P(A \cap B) = P(A) \times P(B)$$ 6. For conditional probability, the probability of $A$ given $B$ is: $$P(A|B) = \frac{P(A \cap B)}{P(B)}$$ These formulas cover fundamental concepts in O Level probability.