Probability
Calculate probability of simple events
What is Probability?
Probability measures how likely something is to happen. It is a number between 0 and 1. A probability of 0 means impossible. A probability of 1 means certain. We can also write probability as a fraction, decimal, or percentage.
Example
Calculating Probability
Probability = number of favourable outcomes / total number of outcomes
Rolling a 4 on a die: favourable = 1, total = 6
P(4) = 1/6
Drawing a red ball from a bag with 3 red and 7 blue balls:
P(red) = 3/10 = 0.3 = 30%
The Probability Scale
0 = Impossible (e.g. rolling a 7 on a standard die)
0.25 = Unlikely
0.5 = Even chance (e.g. flipping heads on a coin)
0.75 = Likely
1 = Certain (e.g. the sun will rise tomorrow)
Example
Experiment: Flipping a Coin
A coin has 2 sides: heads and tails.
P(heads) = 1/2 = 0.5 = 50%
P(tails) = 1/2 = 0.5 = 50%
If you flip a coin 20 times, you would EXPECT about 10 heads and 10 tails — but the actual results may differ!
Note
Remember
All probabilities are between 0 and 1. The probabilities of all possible outcomes must add up to 1. P(event not happening) = 1 − P(event happening).
Key Vocabulary
ProbabilityA measure of how likely an event is to occur
OutcomeA possible result of an experiment
EventA specific outcome or set of outcomes
CertainDefinitely will happen (probability = 1)
ImpossibleCannot happen (probability = 0)
ExperimentAn action with uncertain results, like flipping a coin
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Probability
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