Introduction

How likely something is to happen.

Many events can't be predicted with total certainty. Probability is the likelihood or chance of an event occurring. The best we can say is how likely they are to happen, using the idea of probability.

Probability =	the number of ways of achieving success
	the total number of possible outcomes

For example, the probability of flipping a coin and it being heads is ½, because there is 1 way of getting a head and the total number of possible outcomes is 2 (a head or tail). We write P(heads) = ½ .

The probability of something which is certain to happen is 1.
The probability of something which is impossible to happen is 0.
The probability of something not happening is 1 minus the probability that it will happen.

Another example: there are 5 marbles in a bag: 4 are blue, and 1 is red. What is the probability that a blue marble gets picked?

Number of ways it can happen: 4 (there are 4 blues)

Total number of outcomes: 5 (there are 5 marbles in total)

So the probability = 45 = 0.8

Probability Line

Probability is the chance that something will happen. It can be shown on a line.

The probability of an event occurring is somewhere between impossible and certain.

As well as words we can use numbers (such as fractions or decimals) to show the probability of something happening:

· Impossible is zero

· Certain is one.

Here are some fractions on the probability line:

We can also show the chance that something will happen:

a) The sun will rise tomorrow.
b) I will not have to learn mathematics at school.
c) If I flip a coin it will land heads up.
d) Choosing a red ball from a sack with 1 red ball and 3 green balls