Intermediate · 12 min
Probability of simple events
Count equally likely outcomes and calculate complementary probabilities.
Count favourable outcomes
When outcomes are equally likely, probability is the number of favourable outcomes divided by the total. Probabilities range from 0 to 1.
Worked example
A fair six-sided die has three even faces, so P(even) = 3/6 = 1/2.
Use the complement
The probability that an event does not happen is 1 minus the probability that it does. Include every possible outcome in the total.
Worked example
If P(rain) = 0.3, then P(no rain) = 0.7.
Predict and test
- Predict the probability of getting a number greater than 4 on a fair six-sided die.
- Roll a die several times and compare the observed fraction with the prediction; they need not match exactly.