Probability
The probability scale The chance of an event happening can be shown on a probability scale. impossiblecertaineven chanceunlikely likely Less likelyMore likely The sun won’t rise tomorrow A day of the week starting with a T The next baby born being a girl You will receive a text today A square having four right angles
The probability scale We measure probability on a scale from 0 to 1. If an event is impossible or has no probability of occurring then it has a probability of 0. If an event is certain it has a probability of 1. This can be shown on the probability scale as: Probabilities are written as fractions, decimals and, less often, as percentages between 0 and 1. impossiblecertaineven chance
Listing possible outcomes When you roll a fair dice you are equally likely to get one of six possible outcomes: Since each number on the dice is equally likely the probability of getting any one of the numbers is 1 divided by 6 or. 1 6
Calculating probability If the outcomes of an event are equally likely then we can calculate the probability using the formula: Probability of an event = Number of successful outcomes Total number of possible outcomes For example, a bag contains 1 yellow, 3 green, 4 blue and 2 red marbles. What is the probability of pulling a green marble from the bag without looking? P(green) = 3 10 or 0.3 or 30%
Calculating probability What is the probability of the following events? P(tails) = 1 2 P(red) = 1 4 P(7 of ) = 1 52 P(Friday) = 1 7 2) This spinner stopping on the red section? 3) Drawing a seven of hearts from a pack of 52 cards? 4) Your birthday being on a Friday this year? 1) A coin landing tails up?
Scratch cards You are only allowed to scratch off one square and you can’t see what is behind any of the squares. Which of the scratch cards is most likely to win a prize? £ no win no win no win no win no win ££ no win no win $ no win $ no win no win no win no win no win no win $ no win no win no win no win no win $$ no win no win $ no win no win no win $ no win no win $ Scratch off a $ sign and win $100! no win no win no win no win no win no win
Calculating probability A fair dice is thrown. What is the probability of getting a)a 2? b)a multiple of 3? c)an odd number? d)a prime number? e)a number bigger than 6? f)an integer? a) P(2) = 1 6 b) P(a multiple of 3) = 2 6 = 1 3 c) P(an odd number) = 3 6 = 1 2
Calculating probability A fair dice is thrown. What is the probability of getting a)a 2? b)a multiple of 3? c)an odd number? d)a prime number? e)a number bigger than 6? f)an integer? d) P(a prime number) = 3 6 e) P(a number bigger than 6) = f) P(an integer) = 6 6 = 1 = Don’t write 0 6
Probability Exercise 8.1 – Page 246