math · 28 minutes · Official OETT Aligned
Percentages
Find a part, discount, or change
What to know
- percent: out of 100
- percent of an amount
- increases and decreases
- the starting amount
Percent questions are easier when you identify the starting amount.
The core rule
To find a percent of an amount, change the percent to a decimal and multiply. Percent change = change ÷ starting amount × 100.
Percent means out of 100. For 15% of 240, use 0.15 × 240 = 36. For an increase from 80 to 100, the change is 20. Divide 20 by the starting 80 to get 0.25, or 25%.
Worked examples
Example 1: easy
15% of 240 = 36.
- Convert 15% to 0.15.
- Multiply 0.15 × 240 = 36 and label the result as the requested part.
Percent means out of 100, so 15% = 0.15.
Example 2: medium
An increase from 80 to 100 is 25%.
- Find the change: 100 − 80 = 20.
- Divide by the original 80: 20/80 = 0.25 = 25%.
Compare the change with the amount you started with.
Example 3: hard
After a 20% reduction, a $360 amount becomes $288.
- A 20% reduction leaves 80%, or multiplier 0.80.
- Calculate $360 × 0.80 = $288 and verify the result is below $360.
A 20% discount means you pay the remaining 80%.
Common mistakes
Use the new amount to calculate percent change.
Divide the change by the original amount. That is the amount you started with.
Put it into practice
Use guided explanations, check your answers, and try a new variation.