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# Learner Guide

Probability

We use probability to figure out how likely it is that something will happen.

For example, for people who live in Britain, it is very likely that it will rain in

September. But it you live in the middle of the Sahara Desert, it is very

unlikely that it will rain in September (or any other time of year!).

Many events will have more than one outcome – if you roll a dice, it could

land on 1, 2, 3, 4, 5 or 6 – so we use probability to help us guess which

outcome is going to happen.

If there is an equally likely chance of things happening, then we say that

there is an even chance.

We can use words and fractions to show the probability of something

happening. However, if we’re writing probability as a fraction, it’s important

to remember that these probabilities can only be between 0 and 1. 0 is

impossible and 1 is certain and we can’t have a chance that something is

going to happen as less than impossible or more than certain.

An even chance of two things happening is written as ½ .

 Example 1.  If we use our dice example again, we can show words and fractions can be used to show probability.  Let’s say that we want to find out the probability of rolling a two.    There are six possible outcomes of rolling a dice – that it will land on 1, 2, 3, 4, 5 or     6 – and there’s only one of these outcomes that we can roll two. So, the probability of rolling a two is 1 in 6.    If we want to write this as a fraction, the number of possible outcomes becomes the    denominator and the number of ways to get the outcome we want is the numerator. So, as a fraction, the probability of rolling a two is  1/6.

Therefore, when trying to find probability as a fraction, you must first find all

the different outcomes that could possibly happen, and then find how many

of those different outcomes are the outcome that you are trying to find.

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