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Probability

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Have you ever heard someone say there's a 'fifty-fifty chance' that something might happen? Do you ever describe the chances of something happening as 'one in a million'? Both of these expressions are ways of talking about probability. Probability is a measurement of how likely it is that a particular event will happen. A high probability means something is likely to happen but may not; a low probability means it probably won't but still could.

Suppose you have a black bag that contains three red apples and one green apple. If you reach into the bag and pick out one apple without looking, what is the probability that it will be a green apple? The probability is one in four because, of the four apples you might pick, only one of them is green. This probability can be written as a fraction: 14. Or it can be written as a percentage: there is a 25% chance that you will pick a green apple. Probability can also be expressed as a decimal value between 0 and 1. A probability of 0 means that there is no chance of the event occurring. A probability of 1 means it is certain to occur. In our example, the probability of choosing a green apple is 0.25.

What is the probability of choosing a red apple? Can you write that probability in each of the three different ways?

Now imagine that you have four red apples and no green apples in the bag: what is the probability of pulling out a green one? What is the probability of pulling out a red one?


This activity is adapted from pages 256-7 of What Your Year 6 Child Needs to Know.