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Probability
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Probability- the likelihood that an event will have a particular result; the ratio of the number of desired outcomes to the total possible outcomes.
Event- an experiment where there is an outcome Outcome- a result of an experiment Independent Event- an event to which the
result is in no way affected by other events Dependent Event- an event where the outcome
depends or is affected by the outcome of another event.
Key Terms
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◦ The probability of an impossibility is 0
◦ The probability of a certainty is 1
Probability is always a number between 0 and 1
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The probability that an event will not occur is just
1 – the probability that the event will occur.
The sum of the probabilities of all outcomes of an event is always 1
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To convert from fraction to a decimal
divide the numerator by the denominator
(round if necessary)
2/3 = 2 ÷ 3 = .67
Probability can be written as a fraction or as a decimal
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Also called “experiments” Examples:
◦ Flip a coin◦ Draw a random card◦ Choose a random marble, sock, book
Events
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(also called “results”) Examples:
◦ Heads◦ Ace of Hearts◦ Green marble◦ Red sock◦ “Cat in the Hat”
Outcomes
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List of the possible outcomes (no duplicates)
Examples:◦ For a coin toss: {heads, tails}◦ To choose from a bag of marbles with 2 red
marbles, 4 blue marbles, and one yellow marble, {red, blue, yellow}
Sample Space
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P(E) =
Probability of drawing a jack from a standard
deck of cards = number of jacks in a deck = 4 = 1 number of cards in a deck 52 13
Probability of an Event
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number of red 10’s in a deck = 2 = 1number of cards in a deck 52 26
Probability of drawing a red 10 from a deck of cards
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number of red cards in a deck = 26 = 1number of cards in a deck 52 2
Probability of drawing a red card from a deck of cards
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= number of Aces of Hearts in a deck = 1 number of cards in a deck
52
Probability of drawing an Ace of Hearts from a standard
deck of cards
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To find the probability of multiple independent events, multiply the probabilities of each event together.
Key Facts
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To find the probability of multiple dependent events, multiply the probabilities of each event together considering the dependence.
Key Facts
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Multiplication Rule 2: When two events, A and B, are dependent, the probability of both occurring is:
Conditional Probbility
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The conditional probability of an event B in relationship to an event A is the probability that event B occurs given that event A has already occurred. The notation for conditional probability is P(B|A), read as the probability of B given A.
Conditional Probability
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A jar contains black and white marbles. Two marbles are chosen without replacement. The probability of selecting a black marble and then a white marble is 0.34, and the probability of selecting a black marble on the first draw is 0.47. What is the probability of selecting a white marble on the second draw, given that the first marble drawn was black?
Example 1
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The probability that it is Friday and that a student is absent is 0.03. Since there are 5 school days in a week, the probability that it is Friday is 0.2. What is the probability that a student is absent given that today is Friday?
Example 2
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At Kennedy Middle School, the probability that a student takes Technology and Spanish is 0.087. The probability that a student takes Technology is 0.68. What is the probability that a student takes Spanish given that the student is taking Technology?
Example 3
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Value between 0 and 1
Wins/Total Outcomes
Usually expressed as a fraction or decimal
Any positive number or 0
Wins/Losses (Odds For)or Losses/Wins (Odds Against) Usually expressed
as a ratio
Probability versus Odds