3 Percent Formulas for Daily Life

Formula 1 — ratios

A as a percent of B is A÷B×100. If 45 out of 50 answered, that's 45÷50×100 = 90%.

Use it for response rates, scores, and attendance — any part of a whole.

Formula 2 — percent change

(New − Old)÷Old×100. A raise from $3,000 to $3,300 is (3300−3000)÷3000×100 = 10%.

News numbers about sales or prices use this formula. The denominator is what matters.

Formula 3 — discounts

Price×(1−rate). A $50 item at 20% off is 50×0.8 = $40.

Stack a 10% coupon and it's 40×0.9 = $36. That's why 20%+10% is 28%, not 30%.