Using numbers

When we talk about probability, we don't always use just words to describe how likely something is, sometimes we use numbers too.

You have probably heard someone say "there's a one hundred percent chance", or "fifty-fifty odds", or maybe "a one-in-four chance".

When we use numbers to describe likelihood, there are three types of number we can use: fractions, decimals and percentages. The way we work out the probability is the same for all of them. The probability of an event equals the number of favourable outcomes divided by the total number of possible outcomes.

Fractions and decimals tell us how likely something is to happen out of a total of one. For example, when you toss a coin, you know that there is an equal chance of heads or tails. The probability of tossing a head is 1/2, or 0.5, and so is the probability of tossing a tail. If you add 1/2 + 1/2, or 0.5 + 0.5, you get 1.

Make a list of some examples of things for which you can work out the probability using fractions, such as rolling dice.

Percentages tell us how many times out of one hundred something is likely to happen. So, if you toss a coin, there is a 50% chance of getting heads, and a 50% chance of getting tails. The total of the probabilities of all the things that could happen will always add up to 100%.

Work out the probability of getting a red ball if you take one at random from a bag containing one red and three blue balls. Give this probability as a percentage, then try expressing it as a fraction and a decimal.