The basic question
"What is 30% of 80?" means: if 80 is the whole (100%), how big is the 30% part?
Method 1: Decimal multiplier
Change the percent to a decimal, then multiply.
- 30% = 0.30
- 0.30 × 80 = 24
The decimal (0.30) is called a decimal multiplier. This method works for any percent, especially with a calculator.
Method 2: Fraction
Change the percent to a fraction.
- 25% of 64 = 1/4 × 64 = 16
- 75% of 40 = 3/4 × 40 = 30
Great when the percent is a friendly fraction.
Method 3: Build from benchmarks
Use benchmark percents you can find in your head:
- 10% → divide by 10
- 1% → divide by 100
- 50% → halve
Example: 35% of 60
- 10% = 6, so 30% = 18
- 5% = 3 (half of 10%)
- 35% = 18 + 3 = 21
Example: 15% of $42
- 10% = $4.20; 5% = $2.10
- 15% = $6.30
Method 4: Proportion
Set up a proportion: part/whole = percent/100.
Example: What is 12% of 250?
- x/250 = 12/100
- x = 12 × 250 ÷ 100 = 30
Percents over 100%
- 150% of 40 = 1.5 × 40 = 60
- 200% of 7 = 14 (double)
Percents less than 1%
- 0.5% of 800 = 0.005 × 800 = 4
Swap trick
x% of y = y% of x. So 8% of 50 = 50% of 8 = 4. Handy when one way is easier!
Worked examples
Example 1: 30% of 80
0.3 × 80 = 24.
Example 2: 60% of a class of 30 students wear glasses. How many?
0.6 × 30 = 18 students.
Example 3: 4% of 250
Swap: 250% of 4 = 2.5 × 4 = 10.
Common mistakes
- Using 30 instead of 0.30 when multiplying (getting 2,400).
- Dividing by the percent instead of multiplying.
- Mixing up the part and the whole.
Try it yourself
- 20% of 45
- 75% of 120
- 15% of 80
- 2.5% of 400
- 125% of 16
Answers: 1) 9 2) 90 3) 12 4) 10 5) 20
Quick recap
- Percent of = percent (as a decimal or fraction) × whole.
- Build hard percents from benchmarks like 10% and 5%.
- A proportion works too: part/whole = percent/100.