YOU ARE LEARNING:
What Is Probability?
What Is Probability?
You can express the probability of unknown events on a scale from certain to impossible. You can also express them as fractions or in percent.
What does probability measure?
With a normal 6-sided die, what is the probability of rolling a number less than 7?
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/hg5zf3LlRLe4ZNUOfvhD.png)
Rolling a number smaller than 7 with a normal 6-sided die is certain. How can we express this as a percentage?
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/hg5zf3LlRLe4ZNUOfvhD.png)
An outcome that will always happen is a "certain" probability
A certain probability is the same as a probability of 100%
It is certain that you will roll one of 1, 2, 3, 4, 5 or 6 when you roll a normal 6-sided die. It will happen 100% of the time.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/hg5zf3LlRLe4ZNUOfvhD.png)
We have seen that it is a certain probability that we will roll a number less than 7 on the die. What is the probability that we will roll a 7 or higher?
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/hg5zf3LlRLe4ZNUOfvhD.png)
A certain probability happens 100% of the time. An impossible probability never happens, what percentage is this?
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/hg5zf3LlRLe4ZNUOfvhD.png)
An outcome that will never happen is an "impossible" probability
For example, it is impossible that you will roll a 9 on a 6-sided die. It will happen 0% of the time.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/hg5zf3LlRLe4ZNUOfvhD.png)
This shows us the probability scale from impossible to certain.
In percentage, the scale goes from 0% to 100%.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/hg5zf3LlRLe4ZNUOfvhD.png)
Certain probability is 100% and impossible probability is 0%. Between those two is 50%, what term describes a 50% probability?
There are eight balls in this bag, four are red and four green. Is this statement true or false? If you close your eyes and pull a random ball from this bag, it is more ** likely that the ball is red than that it is green.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/qMltCKMQQqqXbgCwx17K.png)
What is the probability of pulling a green ball from the bag?
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/qMltCKMQQqqXbgCwx17K.png)
So if the probability of something happening or not happening is the same, it's an "even chance"
For example, there is an even chance between randomly pulling a red ball out of this bag and it not being a red ball. In both cases, the probability is 50%
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/qMltCKMQQqqXbgCwx17K.png)
You can put probability on a scale like this
Certain probability is 100% or 1
Even chance is 50% or 21
Impossible probability is 0% or 0
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/28ZUBcsHQ0mmYQ0Uo4wz.png)
This shows the scale for probability from impossible to certain with even chance in the middle. What word describes the section between Even chance and Certain?
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/28ZUBcsHQ0mmYQ0Uo4wz.png)
We can see that a probability between even chance and impossible is unlikely.
However, even if the probability is unlikely it can still happen.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/Qfb2p7pVQymMuGsdQEJK.png)
This bag contains seven balls, six are red and one is green. Which phrase describes the probability of pulling out a ball which is green?
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/OQrpqwyRZyTLTuhT2roD.png)
The probability of pulling out the green ball is unlikely.
We can also say that is it less likely to pull out the green ball than a red ball.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/OQrpqwyRZyTLTuhT2roD.png)
We saw on the probability scale that an even chance can be shown as a percentage (50%) or a fraction (21).
Probabilities can expressed in different ways, but the scale always sits between 0 and 1.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/lMdNPgFPTr6yn3NwIPfv.png)
The bag contains a total of 10 balls. What is the probability of pulling out a green ball as a fraction?
3 are green 3 are purple 2 are yellow 2 are red
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/lMdNPgFPTr6yn3NwIPfv.png)
To find a probability as a fraction.
Work out the number of possible outcomes (in this case three green balls) out of the total number of options (in this case the ten balls) and write as a fraction.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/lMdNPgFPTr6yn3NwIPfv.png)
There are 20 balls in this bag, 15 are yellow and 5 are purple. Which term describes the probability of pulling out a yellow ball?
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/Qhh5VXsnSH2XA8Ll1cmv.png)
With 15 yellow balls and 5 purple, what is the probability of pulling out a yellow ball as a fraction?
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/Qhh5VXsnSH2XA8Ll1cmv.png)
Summary! Probability is a measure of how likely something is to happen
You can express it in words, as fractions and in percent.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/Qfb2p7pVQymMuGsdQEJK.png)
Impossible probability is when something will never happen, so 0% of the time.
It's impossible to roll a 7 with a normal 6-sided die.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/Qfb2p7pVQymMuGsdQEJK.png)
Unlikely probability is when something is less likely to happen or between 0% and 50%.
It's unlikely that you will pull a green ball out of a bag if only 103 balls in the bag are green.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/Qfb2p7pVQymMuGsdQEJK.png)
Likely probability is when something is more likely to happen, or between 50% and 100%.
It's likely that you will pull a purple ball out of a bag if 107 balls in the bag are purple.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/Qfb2p7pVQymMuGsdQEJK.png)
Certain probability is when something will always happen or 100% of the time.
It's certain to roll a number smaller than 7 with a normal 6-sided die.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/Qfb2p7pVQymMuGsdQEJK.png)
Even chance is when something is just as likely to happen as it is to not happen or 50% of the time.
The probability of rolling an even number with a normal 6-sided die is even chance because that will happen 63 or 21 the time.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/Qfb2p7pVQymMuGsdQEJK.png)
To express a probability as a fraction take the number of possible outcomes out of the total number of options.
Here the probability of pulling out a purple ball is 205=41.
![](https://cdn.hejalbert.se/teen/blocks-images/en_GB/Qhh5VXsnSH2XA8Ll1cmv.png)