How_To_Be_Good_At_Math

298 STATISTICS •^ CALCULATING PROBABILITY

Calculating probability

We can use a simple formula to help us work out the probability of

something happening. The formula expresses the probability as a fraction.

We can also change probability fractions into decimals and percentages.

Let’s use the formula below to find

the probability of choosing an apple:

Here’s a box of 12 pieces of fruit. It

contains six apples and six oranges,

randomly arranged. What’s the chance of

picking out an apple if we shut our eyes?

So, we’ve a 6 in 12 chance of picking

an apple. We show this as the

fraction^6 / 12 , which can be simplified to^1 / 2.

We can picture the formula like this.

The top part of the formula means

how many apples it’s possible to take out

of the box (6). The bottom part is the total

number of fruits that could be chosen (12).

number of results we’re interested in

number of all possible results

In all, there are

12 fruits in the box

The probability of

picking an apple

There are six apples in the box

REAL WORLD MATHS

6

12

=

You can write

probabilities as

fractions, decimals,

or percentages.

Unexpected results

Probability doesn’t always tell us exactly what’s

going to happen. There’s a 1 in 6 chance that

this spinner will land on red. If we spin it 6

times, we’d expect to get a red at least once.

But we might get 6 reds – or none.

1

2

=

298_299_Calculating_probability.indd 298 29/02/2016 18:07