Probability rules

Math

Definition

The fundamental laws governing how probabilities combine. The addition rule finds the probability of either event occurring, while the multiplication rule finds the probability of both events occurring.

Examples

  • P(rolling a 2 or 4 on a die) = 1/6 + 1/6 = 2/6
  • P(two heads in a row) = 1/2 × 1/2 = 1/4
  • P(red or face card) = P(red) + P(face) - P(red and face) = 26/52 + 12/52 - 6/52
Key Fact

Addition: P(A or B) = P(A) + P(B) - P(A and B); Multiplication (independent): P(A and B) = P(A) × P(B)

Related Terms

Basic probability Permutations and combinations Binomial probability Conditional statements

Study This Concept

Practice probability rules with free review games in these units:

AP Statistics Unit 4: Probability Random Variables and Distributions