How To Calculate Percentages
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1. X% of Y
Percentages trip people up because there are five different questions they can answer, and the formula changes slightly for each one. “What’s 15% of 80?” is a different beast from “80 is what percent of 320?” or “if the sale price is $60 at 25% off, what was the original?” Here are the five formulas you actually need, each with a worked example and a mental-math shortcut.
2. A is what % of B
Mental-math shortcut: get 10% by moving the decimal one place left (10% of 80 is 8), then scale. 15% is 10% plus half of 10% (8 + 4 = 12). 20% is double 10% (16). 5% is half of 10% (4). Almost any round percentage collapses to this pattern.
3. Percent change
The thing to remember: A goes on top, B on the bottom. “A of B” maps directly to “A over B.” If you’re ever unsure which number is the denominator, it’s the one after “of.”
4. Reverse percent (finding the original)
Before committing to any percentage answer, ask: is this roughly in the right neighborhood? 15% of 80 should be noticeably less than half (40). 30% change on 50 should be around 15. If your calculator spits out 150, you fat-fingered a decimal. This three-second gut check catches 90% of the errors that actually cost money.