A scale from 0 to 1
A jar holds ten glass marbles: five red, three blue, two green. Draw one without looking.
- A line from 0 to 1.
- 0 is impossible. 1 is certain.
- Red is 5 of 10 marbles: its chance is 0.5.
- Blue is 0.3 and green is 0.2. All three add to 1.
Chances are numbers
Each marble is equally likely to be picked. Red's share is 5/10 = 0.5. Blue is 3/10 = 0.3. Green is 2/10 = 0.2.
Probability — a chance written as a number. Write it as a fraction, a decimal or a percentage.
Outcome — one possible result of a draw. Here the outcomes are red, blue and green.
The two ends of the line
Impossible — a draw that cannot happen. Its chance is 0. No yellow marble is in the jar. So yellow's chance is 0.
Certain — a draw that must happen. Its chance is 1. The marble is always red, blue or green. So that chance is 1.
Between the ends we use words: less likely, even chance, more likely. An even chance is 1/2.
| Colour | Marbles in the jar | Chance |
|---|---|---|
| Red | 5 of 10 | 0.5 |
| Blue | 3 of 10 | 0.3 |
| Green | 2 of 10 | 0.2 |
0.5 + 0.3 + 0.2 = 1. One draw gives one colour. The three chances together make 1.
Red is 0.5, so "not red" is 1 − 0.5 = 0.5. Check: blue and green give 0.3 + 0.2 = 0.5.
Rule: A chance is a number from 0 to 1. 0 is impossible. 1 is certain. The chances of all the outcomes of one draw add to 1.
Watch out!
A 1-in-1000 chance is not 0. It is a small number above 0. It can still happen.
The jar below holds 10 marbles. Slide the number of red marbles from 0 to 10. The pointer shows the chance of red on the line. Match each goal.
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