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Calculating a probability.
Probabilities can only have values from zero to one, including these values.
You can be like my granddad. He likes to show probabilities on a probabilities scale. He
uses his washing line.
He shows the probabilities using decimals, fractions and percentages. Using the scale
helps him to remember that probabilities lie between zero and one.
Or zero percent and one hundred percent.
You should use this fact to check your answer when you are calculating probabilities.
If you have an answer less than zero or greater than one; hey I'm sorry you've made a
mistake.
So how do we calculate probabilities?
Take a walk with me and I'll show you.
I went to a shop and bought a bag of sweets. In the bag there were four red sweets five
blue sweets and two green sweets.
Now let me ask you to pick a sweet out-only one mind!
What is the probability of you choosing a green sweet?
To save writing probability of green again we use short hand notation.
We write it this way:
Choosing green in this case is known as the event. This is the thing we are interested
in.
So how many green sweets are there? There are two.
How many sweets are there all together? Well 2+4+5 gives us a total of 11.
So the probability of choosing a green sweet is 2 out of 11. Remembering to give our answer
as a fraction, a decimal or a percentage.
Lets look at another question. What's the probability of choosing a red sweet?
In this case choosing red is the event. There are 4 red sweets and 11 sweets altogether.
So the probability of choosing red is 4 out of 11.
Can you see how we are getting our answers?
In general the probability of an event equals the number of times the event occurs over
the total number of possible outcomes.
This gives the answer as a fraction but you may want to change the fraction to a decimal
or a percentage- depending on what they ask you in the question.
I hope you can now go off and calculate your own probabilities.
Good luck and remember, don't eat too many sweets!