Probability Basics
Probability = favorable outcomes / total outcomes.
The rules
- Probabilities are between 0 and 1.
- P(not A) = 1 − P(A).
- For independent events, multiply.
- For either/or with no overlap, add.
Watch out: Counting the total outcomes wrong.
Worked example
A bag has 3 red and 5 blue marbles. What is the probability of picking a red one?
- Favorable: 3.
- Total: 8.
Answer: 3/8
Try it yourself
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