Pie charts are based on fractions of 360°.
The fractions can be worked out from the angles at the centre of the chart.
This pie chart shows the results of a survey on favourite crisp flavours.
The Salt and Vinegar sector shows 36° which is 36/360 of the whole circle (i.e. 1/10)
Now we need a hard fact, such as the actual number who chose Salt and Vinegar or the total number in the survey.
If 15 people chose Salt and Vinegar, how many people took part in the survey?
15 people are shown in 1/10 of the circle, so there must be 10 x 15 = 150 people in total.
If 40 people took the survey, how many people chose Salt and Vinegar?
We need to work out 1/10 of 40 = 4, so 4 people chose Salt and Vinegar.