Chance and probability · Usually met around P3
Probability as a fraction
Wanted ÷ all.
How to do it
Method
Wanted ÷ all
3 red, 1 blue: chance of red = 34
Another way
Chance scale
0 impossible, 12 even, 1 certain
In full
When every outcome is equally likely, a chance can be written as a fraction: the number of outcomes you want ÷ the number of outcomes in all. With 3 red balls and 1 blue, 4 in all, the chance of red is 34 and of blue 14. An impossible event has chance 0, a certain one 1, and everything else lies between them; an even chance is 12. Or use a chance scale: 0 at the left (impossible), 1 at the right (certain); the further right, the more likely.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
On the chance scale, what chance is the arrow pointing to, as a fraction?
Answer in simplest form
Show the answer and working
Answer: 18
0 to 1 is split into 8 equal steps, and the arrow is 1 steps along: 18.
Example 2
The chance of taking a red ball is 15. Which describes it best?
- A. Red is impossible
- B. Possible, and more than an even chance
- C. Possible, but less than an even chance
- D. An even chance
Choose the right answer
Show the answer and working
Answer: C. Possible, but less than an even chance
Compare 15 with 12 by putting double 1 against 5: less is under half, equal is exactly half, more is over half.
Example 3
A bag holds 1 red balls and some blue balls. The chance of taking red is 114. How many blue balls are there?
Show the answer and working
Answer: 13
114 means 1 red out of 14 balls: blue = 14 − 1 = 13.
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