How to Find Theoretical Probability

When you toss a coin, the two possible outcomes are Heads or Tails. Since each of these two outcomes has a 1 over 2 chance of occurring, they are called equally likely. When you roll a number cube, there are 6 equally likely outcomes: the numbers 1, 2, 3, 4, 5, or 6. Each of those occurs with probability 1 over 6.

If all outcomes are equally likely, you can find the probability of an event using these steps.

  • Find the total number of possible outcomes.
  • Find the number of favorable outcomes for your event.
  • Calculate probability = .

For example, suppose you roll a number cube and want to know the probability of an odd number.

  • number of possible outcomes = 6
  • number of favorable outcomes = 3
  • probability = 3 over 6 = 1 over 2