Addition rule

The general addition rule is $P(A \cup B) = P(A) + P(B) - P(A \cap B)$, where $\cup$ is the union symbol (A or B) and $\cap$ is the intersection symbol (A and B). You subtract the overlap because every outcome sitting in both events was already counted once in $P(A)$ and a second time in $P(B)$. That subtraction disappears only when the two events are mutually exclusive, because then they share no outcomes and $P(A \cap B) = 0$. Drawing one card from a standard 52-card deck, $P(\text{heart or king}) = 13/52 + 4/52 - 1/52 = 16/52 \approx 0.3077$.