What you need to know
Probability measures how likely something is to happen. It is always a number between 0 (impossible) and 1 (certain). You can write it as a fraction, a decimal or a percentage: an even chance is 12 = 0.5 = 50%.
When every outcome is equally likely, use the rule: probability = number of favourable outcomes ÷ total number of outcomes. A favourable outcome is one that counts as the event you are interested in.
| Word | Probability |
|---|
| Impossible | 0 |
| Unlikely | between 0 and 12 |
| Even chance | 12 (0.5 or 50%) |
| Likely | between 12 and 1 |
| Certain | 1 |
Rolling a standard six-sided die
- There are 6 equally likely outcomes: 1, 2, 3, 4, 5, 6.
- P(rolling a 4) = 16, because only one outcome is a 4.
- P(rolling an even number) = 36 = 12, because 2, 4 and 6 are even.
- P(rolling a number less than 3) = 26 = 13, because 1 and 2 count.
- P(rolling a 7) = 0. It is impossible.
A bag holds 3 red, 5 blue and 2 green marbles. One is picked without looking.
- Total marbles: 3 + 5 + 2 = 10.
- P(blue) = 510 = 12 = 0.5 = 50%.
- P(red or green) = 310 + 210 = 510 = 12.
- P(not red) = 710 = 0.7 = 70%, because 7 of the 10 marbles are not red.
A spinner has red, blue and yellow sections. P(red) = 0.3 and P(blue) = 0.45. Find P(yellow).
- The probabilities of all the possible outcomes add up to 1.
- P(yellow) = 1 − 0.3 − 0.45 = 0.25.
- Check: 0.3 + 0.45 + 0.25 = 1.
- The same idea gives the chance that something does not happen: P(not red) = 1 − 0.3 = 0.7.
Expected results. If you repeat an experiment many times, the number of times you expect an event is probability × number of trials. Flip a coin 50 times and you expect about 25 heads. The real result might be 22 or 28; that is normal. Results from an actual experiment are called experimental probability, while the calculated value is theoretical probability. The more trials you do, the closer they usually get.
A die is rolled 60 times. How many sixes would you expect?
- P(six) = 16.
- Expected number of sixes = 16 × 60 = 60 ÷ 6 = 10.
- If you actually rolled 8 sixes, that is not wrong; it is normal variation.
Common mistake: thinking a result is "due". If a coin lands on heads four times in a row, the chance of heads on the next flip is still 12. The coin has no memory.
Common mistake: counting outcomes without checking they are equally likely. A footy game has two outcomes, win or lose, but that does not make the chance of winning 12. The formula only works when every outcome has the same chance, like the faces of a fair die.
Check yourself
- What is the probability of rolling a 5 on a standard die?
- What is the probability of rolling an odd number on a standard die?
- A bag holds 4 red and 6 yellow lollies. Write P(yellow) as a fraction, a decimal and a percentage.
- The chance of rain tomorrow is 0.35. What is the chance it does not rain?
- A card is drawn from a standard pack of 52 playing cards. What is P(heart)?
- A spinner with 4 equal sections numbered 1 to 4 is spun 80 times. How many times would you expect a 3?
Answers: 1. 16 2. 36 = 12 3. 610 = 35, 0.6, 60% 4. 0.65 5. 1352 = 14 6. 20 (because 14 × 80 = 20)