Percent vs percentage points
When a rate goes from 4% to 5%, it rose by 1 percentage point, but by 25 percent, because 1 is a quarter of 4. News about interest rates, tax rates and polls usually means percentage points. If someone says “up 1%” about a rate, check which one they mean.
Why +10% then −10% does not get you back
100 increased by 10% is 110. Taking 10% off 110 gives 99, not 100, because the second 10% is worked out on the bigger number. To undo an increase, divide by the same factor instead: 110 ÷ 1.1 = 100. That is what the reverse calculator does.
Doing it in your head
10% is the number with the decimal point moved one place left: 10% of 240 is 24. From there, 5% is half of that (12), 20% is double (48) and 15% is 10% plus 5% (36). For a tip or sales tax that is often close enough; for money that has to be exact, use the calculator.