What you need to know
Per cent means 'out of 100'. So 45% is 45 out of every 100, which is the fraction 45100 and the decimal 0.45. Percentages are useful because they make it easy to compare amounts: 45% of a small class and 45% of a big school are the same share, even though the numbers are different. 100% means the whole amount.
| Fraction | Decimal | Percentage |
|---|
| 12 | 0.5 | 50% |
| 14 | 0.25 | 25% |
| 34 | 0.75 | 75% |
| 110 | 0.1 | 10% |
| 15 | 0.2 | 20% |
| 1100 | 0.01 | 1% |
| 1 | 1.0 | 100% |
Converting
- Fraction → percentage: make the denominator 100. 35 = 60100 = 60%.
- Decimal → percentage: multiply by 100 (move the digits two places left). 0.35 = 35%.
- Percentage → decimal: divide by 100 (move the digits two places right). 8% = 0.08.
- Percentage → fraction: write it over 100 and simplify. 40% = 40100 = 25.
Common mistake: writing 8% as 0.8. Remember 8% is only 8 hundredths, so it is 0.08. (0.8 would be 80%.)
Finding a percentage of an amount. You do not need a calculator for most percentages. Start with an easy percentage and build up: 10% is the amount divided by 10, 1% is the amount divided by 100, 50% is half and 25% is a quarter.
Three mental methods: 30% of 80, 15% of 60, 12% of 250
- 30% of 80: 10% of 80 = 8, and 30% is three lots of 10%, so 3 × 8 = 24.
- 15% of 60: 10% of 60 = 6, and 5% is half of that, 3. So 15% = 6 + 3 = 9.
- 12% of 250: 1% of 250 = 250 ÷ 100 = 2.5, so 12% = 12 × 2.5 = 30.
A $40 jacket is 25% off. What do you pay?
- 25% is a quarter. 40 ÷ 4 = 10, so the discount is $10.
- Take the discount off the price: 40 − 10 = 30.
- You pay $30.
Read the question carefully: 'How much is the discount?' and 'What is the new price?' have different answers. In the jacket example the discount is $10 but the price is $30.
Check yourself
- Write 0.6 as a percentage.
- Write 720 as a percentage.
- 20% of 45 = ?
- 75% of 120 = ?
- 35% of 200 = ?
- A $50 game is 30% off. What is the new price?
Answers: 1. 60% 2. 35% (720 = 35100) 3. 9 4. 90 5. 70 6. $35 (the discount is $15)