Mutually Exclusive Events – The Addition Rule (Leaving Cert Mathematics): Flashcards

Events that cannot occur at the same time

No outcomes in common

$P(A \text{ or } B) = P(A) + P(B)$

OR rule

$P(A \text{ or } B) = P(A) + P(B) - P(A \text{ and } B)$

To avoid counting outcomes twice (double counting)

Can events happen simultaneously? If no, they're mut. excl.

Probability of at least one event happening

Yes - cannot show both 5 and 3 at same time

No - 6 is both even and greater than 4

No - 2 is both prime and even

Double counting outcomes that belong to both events