Percent change is the difference divided by the original amount, then turned into a percent. That is not the same job as “what is 20% of 80,” which is covered in percent problems.
The formula
Percent change = (new − original) / original × 100. A score that moves from 24 to 30 changed by (30 − 24) / 24 = 0.25, which is a 25% increase. From 30 back to 24 is (24 − 30) / 30 = −0.2, a 20% decrease. The denominator is always where you started, not the later number and not the difference.
“Percent of” questions skip that subtraction. 15% of 60 is 0.15 × 60. If the wording is “increased by” or “decreased by,” you are in percent change, not percent of.
A neighbor that also uses weights
Averages that are not a simple mean use a different setup, in weighted averages. Both formulas belong on the list in ACT Math formulas. The mean itself is in mean, median, and mode.
The bottom line
Subtract, divide by the starting value, then multiply by 100. The starting value is the denominator.
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