ACT Science Percent Change Calculations: Avoid Errors on This Common Question Type

Percent change=(New-Old)/Old×100%. This formula works for increases, decreases, and anything in between. Example: If temperature increased from 20°C to 25°C, percent change=(25-20)/20×100%=5/20×100%=25%. Another example: If population decreased from 100 to 80, percent change=(80-100)/100×100%=(-20)/100×100%=-20%. The negative indicates a decrease. This single formula is all you need for any percent change question on the ACT. Once you plug in the numbers and calculate, you're done.

Key insight: The "Old" value (starting value) is always the denominator. If a quantity started at 10 and ended at 12, the change is 2, but the percent change is 2/10×100%=20%, not 2/12×100% (which would be wrong). This denominator choice trips up many students.

Mistake 1: Using the new value as the denominator instead of the old value. "Percent change=(New-Old)/New" is wrong. Always use the original/starting value as the denominator. Mistake 2: Forgetting to multiply by 100% if the question asks for a percentage. If you get 0.25, that's 0.25×100%=25%, not 0.25%. Mistake 3: Misidentifying which is "old" and which is "new." Read the question carefully: "From 2020 to 2021" means 2020 is old and 2021 is new. If you reverse them, your answer will be opposite in sign and magnitude.

Create a reference card with the formula and two worked examples (one increase, one decrease). Reference it daily this week until you can apply the formula without looking.

Problem 1: Mass changed from 50 kg to 60 kg. Percent change=(60-50)/50×100%=10/50×100%=20%. Problem 2: Temperature decreased from 30°C to 24°C. Percent change=(24-30)/30×100%=(-6)/30×100%=-20%. Problem 3: A quantity went from 200 to 250. Percent change=(250-200)/200×100%=50/200×100%=25%. Problem 4: Population decreased from 1000 to 950. Percent change=(950-1000)/1000×100%=(-50)/1000×100%=-5%. Problem 5: A value increased from 15 to 18. Percent change=(18-15)/15×100%=3/15×100%=20%. Solve all five, showing each step of the formula and clearly identifying old and new values.

Find five percent change questions from a practice test. Apply the formula to each. By the fifth question, you should calculate percent change accurately and quickly without referring to notes.

Percent change calculations appear regularly on ACT Science, particularly in data passages that compare values over time or between conditions. Students who master the percent change formula pick up 1 easy point because the calculation is mechanical and the formula is universal. Many students fumble these questions by second-guessing which value goes where or forgetting to multiply by 100%.

Drill the percent change formula this week. Every time you encounter a question asking for percent change, apply the formula without hesitation. By test day, you should calculate any percent change in under 30 seconds.