Percent increase and decrease
Percent change is the change divided by the original amount; a discount multiplies the price by one minus the rate.
A worked example
A price falls from $75 to $60. The change is $15, so the decrease is 15 ÷ 75 = 0.20, which is 20%.
The mistake people actually make
Divide by the original amount, not the new one. Two discounts do not add: 15% then 20% off leaves 0.85 x 0.80 = 0.68 of the price, a 32% cut, not 35%.