There are three main situations you’ll run into, and each has its own quick formula.
Finding what percent one number is of another. Divide the part by the whole, then multiply by 100. If you scored 42 out of 60 on a test: 42 ÷ 60 = 0.7, then 0.7 × 100 = 70%.
Finding a percentage of a number. Convert the percentage to a decimal, then multiply. To find 15% of 80: 15 ÷ 100 = 0.15, then 0.15 × 80 = 12. A faster shortcut for common percentages: 10% is simply moving the decimal point one place left (10% of 80 = 8), and 5% is half of that.
Finding the whole when you know the part and the percentage. Divide the part by the percentage (as a decimal). If you paid $45 in tax and that was 6% of the total price: 45 ÷ 0.06 = $750.
For quick mental math, start with 10% and adjust. Need 20% of $65? 10% is $6.50, double that to get $13. Need a 15% tip on a $48 bill? 10% is $4.80, half of that (5%) is $2.40, add them together for $7.20.
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