Chance and probability · Usually met around P3
Probability: more or less likely
Count them: more is more likely, the same is equally likely.
How to do it
Method
Count them: more is more likely, the same is equally likely
5 red, 2 blue: red is more likely
Another way
Imagine many tries
70 tries: about 50 red, 20 blue
In full
To compare chances, first count how many there are of each: with more red balls than blue, red is more likely and blue less likely; with the same number, the chances are equal. On a spinner with equal parts, count how many parts each colour has. Only the numbers count, not how big the balls are or where they sit; the colour with fewer still has a chance, just a smaller one. Or imagine taking a ball many times, putting it back each time: the colour with more balls comes up more often.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
A ball is taken from the bag without looking. Which colour is it most likely to be?
- A. Yellow 🟡
- B. Green 🟢
- C. Red 🔴
- D. Blue 🔵
Choose the right answer
Show the answer and working
Answer: D. Blue 🔵
Count: 4 red, 6 blue, 3 yellow. The colour with the most balls is the most likely.
Example 2
What is the fewest blue balls to put in so that red 🔴 and blue 🔵 become equally likely?
Show the answer and working
Answer: 6
The bag has 8 red and 2 blue. Equal chances need equal numbers: put in 8 − 2 = 6 blue.
Example 3
A ball is taken without looking from bag A or bag B. Which bag gives a better chance of red 🔴?
- A. Bag B
- B. You can only tell by trying
- C. The chances are the same
- D. Bag A
Choose the right answer
Show the answer and working
Answer: A. Bag B
Both bags have 2 red; the bag with fewer blue balls has red as a bigger share: A has 4 blue, B has 2.
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