How to Calculate Rules of Probability

Calculating the Probability of a Single Event ("A") out of Equally Likely Events

• 1

  Determine how many ways the specific event can occur. For example, when rolling two dice there are three possible combinations that can result in rolling a "10" (5-5, 4-6, and 6-4).

• 2

  Determine how many possible outcomes there are in all. In the example of rolling two dice, there are 36 possible outcomes (1-1, 1- 2, 1-3, and so on).

• 3

  Divide the number of ways your event can occur by the total number of outcomes to determine the probability. In the above example, divide 3 by 36 to determine that the probability of rolling a "10" when throwing 2 dice is 1/12 or .083).

Calculating the Probability of a Compound Event ("A or B"): The Addition Rule

• 4

  Determine the probability of each separate event occurring, using the rules in Section 1, and add the probabilities together.

• 5

  Determine the probability of both events happening together, if that is a possibility.

• 6

  Subtract the probability of both events happening together (the answer in Step 2) from the combined probabilities of each event happening interdependently (the answer in Step 1) to determine the probability of event "A or B" occurring. The mathematical formula that summarized this is P(A or B) = P(A) + P(B) - P(A+B).