Why do people use different methods to find percentages? Which one’s right?

I’ve noticed that some people find percentages by multiplying by decimals (like 0.2 for 20%), while others do it using fractions or ratio steps.

For example, if I’m finding 15% of 80, one person says “multiply by 0.15”, another says “divide by 100 then multiply by 15”, and another just estimates it mentally.

Is there a “best” way to do it, or does it depend on the situation? I want to understand which method is most reliable.

3 Responses

  1. All the methods are the same idea in different clothes. “p% of x” means (p/100)×x. So 15% of 80 can be done as 0.15×80, or (15/100)×80 = (3/20)×80, or by splitting into 10% + 5% (8 + 4). They all give 12. The only time they differ in outcome is when rounding creeps in (for example, 33⅓% is exactly 1/3 but has a repeating decimal).

    Pick the method that fits the numbers and your goal. On a calculator, multiplying by the decimal is usually quickest. For exact work, fractions are safer when the decimal doesn’t terminate (1/3, 2/3, 1/6, etc.). For mental maths, use ratio steps: find 10%, 5%, 1% and combine, or use simple fraction equivalents like 25% = 1/4, 12.5% = 1/8. A quick check helps: 15% of 80 should be between 10% (8) and 20% (16), so 12 makes sense.

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