Fractions, decimals and percentages
- Percent means "out of 100". Learn the common ones: 1/2 = 0.5 = 50%, 1/4 = 0.25 = 25%, 3/4 = 0.75 = 75%, 1/5 = 0.2 = 20%, 1/10 = 0.1 = 10%, 1/8 = 0.125 = 12.5%, 1/3 ≈ 0.333 = 33⅓%.
- Decimal to percentage: multiply by 100 (0.35 = 35%). Percentage to decimal: divide by 100 (7% = 0.07).
Percentages of amounts
- 10% = ÷ 10, 1% = ÷ 100, 50% = ÷ 2. Build others: 15% of 80 = 10% (8) + 5% (4) = 12.
- With a calculator, change the percentage to a decimal: 15% of 80 = 0.15 × 80 = 12.
- One number as a percentage of another: 12 out of 40 = 12 ÷ 40 × 100 = 30%.
Percentage change
- Increase by 20%: multiply by 1.2. Decrease by 15%: multiply by 0.85.
- A £40 jacket reduced by 25% costs 40 × 0.75 = £30.
Decimal calculations
- To add or subtract, line up the decimal points: 2.5 + 0.75 = 3.25.
- To multiply, ignore the points, multiply, then put the point back with the same total number of decimal places: 0.3 × 0.2 = 0.06.
Key terms
- Percentage
- A number out of 100.
- Decimal
- A number with a decimal point, such as 0.75.
- Multiplier
- The number you multiply by to find a percentage change, such as 1.2.
- Percentage increase
- Adding a percentage of an amount onto it.
- Percentage decrease
- Taking a percentage of an amount away from it.
- Equivalent
- Having the same value, such as 0.5 and 50%.