Topic Page: percentage
Way of representing a number as a fraction of 100. For example, 45 percent (45%) equals 45/100, and 45% of 20 is 45/100 × 20 = 9.
Percentage increase/decrease In general, if a quantity changes from one value to another then
percentage = 100 × (difference in the values)/old value
For example, in a sale the price of a bicycle is reduced from £120 to £90, that is there is a discount of £30. The percentage decrease is 100 × 30/120 = 25%.
Fractions as percentages To express a fraction as a percentage, its denominator must first be converted to 100. For example, value-added tax (VAT) as a fraction is 7/40:
7/40 = 17.5/100 = 17.5%
The use of percentages often makes it easier to compare fractions that do not have a common denominator (a number divisible by both the bottom numbers), for instance when comparing rates of inflation or rates of simple interest.
To convert a fraction to a percentage on a calculator, divide the numerator by the denominator and then multiply by 100. The percentage will correspond to the first two figures of the decimal; for example, 7/12 = 0.5833333 = 58.3% (correct to 1 decimal place), and 7/32 = 0.21875 = 21.9% (correct to 1 decimal place).
The percentage sign is thought to have been derived as an economy measure when recording in old counting houses; writing in the numeric symbol for 25/100 would take two lines of parchment, and hence the ‘100’ denominator was put alongside the 25 and rearranged to ‘%’.
essays
Working out percentage increases
Calculating with fractions, decimals and percentages
Calculating with weights
Working out fractions, percentages and ratios
Calculating with percentages
Calculating ratios
Calculating a percentage increase
Working out percentages
Pie charts
Sold! – Percentages Without a Calculator
Standard form: calculating areas and population density
weblinks
BEATCALC: Beat the Calculator
S-Cool! GCSE Maths Revision Guide
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