Percent of a number, ratios and percent change
X × Y ÷ 100X ÷ Y × 100(new − original) ÷ original × 10015% off a 50 item saves 7.50, so it costs 42.50. A price that goes from 80 to 100 rose by 25%. Going back from 100 to 80 is a 20% decrease, not 25%, because the starting value is different.
If a rate rises from 2% to 3%, the percent change is a 50% increase, while the difference is 1 percentage point. Use percentage points when comparing two percentages.
Sale price = original price × (100 − discount %) ÷ 100. A 15% discount on 50 is 7.50 off, so you pay 42.50.
Multiply the amount by (100 + rate) ÷ 100. For an 18% tip on 40, that is 40 × 1.18 = 47.20.