The percent base trap
A free TEAS® math explainer with an original practice question
Percent change compares a change with the amount you started from. Find the change, divide by the original amount, then multiply by 100.
The trap is dividing by the new amount, or by whichever number looks easiest. The base decides the answer: a rise from $64 to $80 is a 25% increase, but a drop from $80 to $64 is a 20% decrease, even though both changes are $16.
Before you divide, say which amount came first. That amount is the base.
Try a sample question
A reusable bag cost $64 last month and costs $80 this month. What is the percent increase relative to last month’s price?
- A.20%
- B.25%
- C.80%
- D.125%
Show the answer
B. 25%
25% is the answer. The price rose by $16. Divide the $16 increase by the $64 original price to get 0.25, or 25%.
Why the other answers tempt:
- A. The $16 increase is 20% of the new $80 price. The question asks for the increase relative to the old price.
- C. $64 is 80% of $80, but this compares the old price with the new price rather than the increase with the old price.
- D. $80 is 125% of $64. That is the new price as a percentage of the old price, not the increase.
Use the original amount as the base for percent change.
Checked against OpenStax Prealgebra 2e, 6.5 Solve Proportions and their Applications; OpenStax Prealgebra 2e, 6.2 Solve General Applications of Percent. Original question, not from a real test.
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