Basic Probability Rules

Start with the fundamentals. $P(A \cup B) = P(A) + P(B) - P(A \cap B)$ for the union of events. $P(A \cap B) = P(A) \cdot P(B)$ only if events are independent.

The complement rule: $P(A^c) = 1 - P(A)$. If you hesitate on these, you'll waste precious time in interviews. Practice until they're automatic.