MEPX
Chapter 6 of 11All chapters
  1. 1What probability means
  2. 2Counting outcomes
  3. 3Independent events
  4. 4Dependent events
  5. 5Complements and at least one
  6. 6Adding probabilities
  7. 7Permutations and combinations
  8. 8Conditional probability
  9. 9Expected value
  10. 10Distributions in outline
  11. 11Fallacies and traps

Chapter 6 of 11

Adding probabilities

Or, and the overlap you must subtract.

Mutually exclusive

If two events cannot both happen, the chance of either is the sum. Rolling a 1 or a 2 is 1 in 6 plus 1 in 6, which is 1 in 3.

  • The probabilities of all possible outcomes always add to 1.
  • Check exclusivity before adding, or you will double count.

When they overlap

For events that can both happen, add the two and subtract the overlap. Drawing a heart or a king is 13 plus 4 minus 1, which is 16 in 52.