Chance and probability · Usually met around P3
Certain, possible, impossible
All of them: certain. None: impossible. Some: possible.
How to do it
Method
All of them: certain. None: impossible. Some: possible
3 red, 2 blue: red is possible, green is impossible
Another way
Ask two questions
Can it happen? Can it not? Both: possible
In full
Something that happens every time is certain; something that happens sometimes but not always is possible; something that can never happen is impossible. First look at what is in the bag or on the spinner: if every ball is red, taking a red ball is certain; if there is no green ball at all, taking a green one is impossible; if there are red and blue balls, taking a red one is possible. Or ask yourself two questions: “Can it happen?” and “Can it not happen?” Yes to both means possible; yes only to the first means certain; yes only to the second means impossible.
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 of these is impossible?
- A. Taking a yellow ball 🟡
- B. Taking a red ball 🔴
- C. Taking a green ball 🟢
- D. Taking a blue ball 🔵
Choose the right answer
Show the answer and working
Answer: B. Taking a red ball 🔴
A colour with no balls in the bag can never be taken: impossible. Every colour in the bag is possible.
Example 2
Spin this spinner. It is impossible for it to land on yellow 🟡.
True or false?
Show the answer and working
Answer: True
Look at the spinner: if any part is yellow, landing on yellow is possible; only with no yellow at all is it impossible.
Example 3
A bag holds 5 red, 4 blue and 2 green balls. What is the fewest balls you must take out so that taking a red ball becomes certain?
Show the answer and working
Answer: 6
Only red balls may be left: all 4 blue and 2 green must go, 6 in all; the red ones stay.
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