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Probability is the 'chance' of something happening.
It is almost certain to come up in your exam! (Nothing is ever completely certain!)
1. We call 'something happening' an event. For example, getting a 6 with the roll of a dice.
2. Probability should always be written as a fraction, decimal or percentage never '1 in 10' or '3 chances out of 5'.
3. The probability of something happening must be between 0 and 1 (unlessyou are using percentage - 0 to 100).
4. The sum of the probabilities of every possible outcome is 1.
5. The probability of something not happening is 1 minus the probability of it happening.
If we call a particular event 'A' then the probability of A happening is:
p (getting an even number when you roll a dice) =
And p (not A) = 1 - p (A)
If p (rolling a 6 on a dice)
Then p (not rolling a 6)
In practice, a coin will not always land 'Heads' exactly half the times you throw it (although theoretically it should!).
If you do something a certain number of times and record the results you can write down estimates for the probabilities of each outcome.
These estimates are also known as Relative Frequencies.
Here's an example:
A ball was drawn out of a bag containing coloured balls 200 times. Each time the ball was replaced. The results are shown below...
Work out all the probabilities and write them in the boxes:
You don't need to bother cancelling the fraction down to its simplest terms in this type of question. You will still get full marks.
Why take the risk?