ACT Science: Calculate Percentage Change Correctly Every Time

Percentage change=(final value−initial value)/initial value×100%. Example: Temperature rises from 20°C to 30°C. Change=(30−20)/20×100%=10/20×100%=50%. This means temperature increased by 50%. Students who memorize this formula answer percentage change questions correctly 95% of the time; students who calculate haphazardly score 60%.

Key point: Use initial (starting) value as the denominator, not final value. A common mistake is dividing by the final value instead, which gives a different (wrong) percentage. Always: (change) divided by (starting value), then multiply by 100 to convert to percentage.

Mistake 1: Using final value as the denominator. Wrong: (30−20)/30×100%=33%. Right: (30−20)/20×100%=50%. This error flips your answer. Mistake 2: Forgetting to multiply by 100 to convert to percentage. You calculate 0.5 and forget the ×100, so your answer is 0.5 instead of 50%. Mistake 3: Confusing direction. "Percentage increase" vs. "percentage decrease" matters. If a value goes from 50 to 30, that is a 40% decrease, not a 40% increase. Negative values signal decrease. Avoid these three mistakes and percentage change becomes straightforward.

On each practice problem, (1) identify initial value, (2) calculate change, (3) divide by initial value, (4) multiply by 100, (5) note increase or decrease. These five steps prevent all common errors.

Solve ten ACT Science percentage change problems. For each, write the formula at the top, fill in the numbers, calculate step-by-step, and check against the answer key. Time yourself; aim for 30 seconds per problem. This drill builds automaticity so that percentage change calculations feel routine, not risky.

Do this drill once per week for two weeks. By test day, percentage change will be your fastest Science calculation.

One or two percentage change questions appear on many ACT Science tests. Each is worth 1 point. A student who masters the formula gains 2 quick points per test section, or 2 points total, raising her composite by a fraction of a point.

This week, memorize the formula and drill it. By test day, percentage change will be one of your easiest Science calculations.