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Learn how to calculate theoretical probabilities for equally likely outcomes with examples of rolling dice and selecting cards from a deck. Discover essential probability facts and the sum of probabilities equaling 1.
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What You Will Learn • Equally Likely Outcomes • Theoretical Probability
Equally Likely Outcomes • If each outcome of an experiment has the same chance of occurring as any other outcome, they are said to be equally likely outcomes. • For equally likely outcomes, the probability of Event E may be calculated with the following formula.
Example 1: Determining Probabilities • A die is rolled. Find the probability of rolling • a) a 5. • b) an even number. • c) a number greater than 3. • d) a 7. • e) a number less than 7.
Example 1: Determining Probabilities Solution a) b) Rolling an even number can occur in three ways: 2, 4 or 6.
Example 1: Determining Probabilities • Solution • c) Three numbers are greater than 3: 4, 5 or 6.
Example 1: Determining Probabilities Solution d) No outcomes will result in a 7. Thus, the event cannot occur and the probability is 0.
Example 1: Determining Probabilities • Solution • e) All the outcomes 1 through 6 are less than 7. Thus, the event must occur and the probability is 1.
Important Probability Facts • The probability of an event that cannot occur is 0. • The probability of an event that must occur is 1. • Every probability is a number between 0 and 1 inclusive; that is, 0 ≤ P(E) ≤ 1. • The sum of the probabilities of all possible outcomes of an experiment is 1.
The Sum of the Probabilities Equals 1 • P(A) + P(not A) = 1 • or • P(not A) = 1 – P(A)
Example 3: Selecting One Card from a Deck • A standard deck of 52 playing cards is shown.
Example 3: Selecting One Card from a Deck • The deck consists of four suits: hearts, clubs, diamonds, and spades. Each suit has 13 cards, including numbered cards ace (1) through 10 and three picture (or face) cards, the jack, the queen, and the king.
Example 3: Selecting One Card from a Deck • Hearts and diamonds are red cards; clubs and spades are black cards. There are 12 picture cards, consisting of 4 jacks, 4 queens, and 4 kings. One card is to be selected at random from the deck of cards. Determine the probability that the card selected is
Example 3: Selecting One Card from a Deck • a) a 7. • b) not a 7. • c) a diamond. • d) a jack or queen or king (a picture card). • e) a heart and spade. • f) a card greater than 6 and less than 9.
Example 3: Selecting One Card from a Deck • Solution • a) a 7. There are 4 7’s in a deck of cards. • b) not a 7.
Example 3: Selecting One Card from a Deck • Solution • c) a diamond. • There are 13 diamonds in the deck.
Example 3: Selecting One Card from a Deck • Solution • d) a jack or queen or king (a picture card). • There are 4 jacks, 4 queens, and 4 kings or a total of 12 picture cards.
Example 3: Selecting One Card from a Deck • Solution • e) a heart and spade. • The word and means both events must occur. This is not possible, that one card is both, the probability = 0.
Example 3: Selecting One Card from a Deck • Solution • f) a card greater than 6 and less than 9. • The cards that are both greater than 6 and less than 9 are 7’s and 8’s. There are 4 7’s and 4 8’s, or 8 total.