Probability describes how likely something is to happen, it can be written as a fraction, a decimal or as a percentage.

The probability scale runs from 0 to 1

0 represents something that is impossible

If something has a probability of 1 it is certain to happen

Here we have a fair spinner. (We say the spinner is fair if the chance of landing on each side is equal.)

There are 4 sides, one of the sides has an A on it. We can say the probability of the spinner landing on A is

If we mark this on the probability scale, it will be

The probability of the spinner landing on C is also

2 of the 4 sides have a B on. We can say the probability of the spinner landing on B is

If we mark this on the probability scale, it will be

The probability of getting any letter other than A, B or C is 0. It would be impossible for the spinner to land on D. We can mark D on the probability scale at 0.

**Example 1: In a bag there are 5 red counters, 4 blue counters and 3 yellow countersA counter is picked at random find the probability that the counter will bea) Redb) Bluec) Yellow**

The total number of counters in the bag = 5 + 4 + 3 = 12

a) 5 out of 12 counters are red

the probability of picking a red counter will be

b) 4 out of 12 counters are blue, the probability of picking a blue counter will be

c) 3 out of 12 counters are yellow, the probability of picking a yellow counter will be

Because the counter can only be red or blue or yellow the three probabilities must add up to 1

**The probabilities of all possible events will always add to 1 (or 100%)**

**Example 2: The probability of it raining tomorrow is 60%. Work out the probability of it not raining tomorrow.**

The probability of it raining and the probability of it not raining must add up to 1 (or 100%)

100% - 60% = 40%

The probability of it not raining = 40%

**Example 3: The probability of Roger winning a tennis match is 7⁄ 12. Work out the probability of Roger not winning the tennis match.**

The probability of Roger winning and the probability of Roger not winning must add up to 1

1 -

The probability of Roger not winning =

**Example 4: There are 20 pens in a box.8 of the pens are black3 of the pens are green.The rest of the pens are red.**

**One of the pens is chosen at randomFind the probability that the pen is red.**

We can work out how many red pens there are by calculating

20 - 8 - 3

20 - 8 - 3 = 9

There are 9 red pens and there are 20 pens in total

The probability of getting a red pen =

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