Percent – Increase and Decrease
Percent change tells you how much a value grew or shrank relative to where it started.
This is a free explanation of Percent – Increase and Decrease — a preview, not the full interactive lesson. The lesson, practice, quizzes, and AI Tutor are inside the app.
Percent change measures how much a value grew or shrank relative to its starting amount. The formula compares the size of the change to the original: percent change = (new − old) ÷ old × 100.
If the result is positive, the value increased; if negative, it decreased. You divide by the original because the same $20 change means something very different if the starting price was $40 versus $400.
A common trap: back-to-back percent changes do not cancel. A price that goes up 10% and then down 10% does not return to the original, because each percent applies to a different base.
Worked example
A price went from $80 to $100. What is the percent increase?
- Find the change: 100 − 80 = 20.
- Divide by the original (where you started): 20 ÷ 80 = 0.25.
- Convert to a percent: 0.25 × 100 = 25% increase.
- Check: 25% of $80 = $20, and $80 + $20 = $100.
25% increase
This concept also comes up in Data Analysis, Word Problems.
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