Probability · Grade 7
In Grade 7 you learn: the probability scale, probabilities as fractions, and listing outcomes.
Probability measures how likely something is, on a scale from 0 (impossible) to 1 (certain). You can work it out from equally likely outcomes. Or you can estimate it from an experiment, using relative frequency.
Key facts
- P(event) = number of favourable outcomes ÷ total number of equally likely outcomes.
- P(not A) = 1 − P(A).
- The probabilities of all the possible outcomes add up to 1.
- Expected frequency = probability × number of trials.
- Mutually exclusive events: P(A or B) = P(A) + P(B).
- Independent events: P(A and B) = P(A) × P(B).
Worked examples
Example 1
A playlist has 8 pop songs, 5 rock songs and 2 jazz songs, played on shuffle. Find P(rock) and P(not jazz) for the first song.
Answer: P(rock) = 1/3, P(not jazz) = 13/15
Example 2
A bus is early with probability 0.15 and on time with probability 0.6. Find P(late), and how many late days to expect in 40 school days.
Answer: 0.25; 10 days
Example 3
Two fair dice are rolled and the scores added. Find the probability of a total of 8.
Answer: 5/36