Calculate the sale price after a percentage off, with optional tax and stacked discounts.
To find the sale price after a percentage discount, multiply the original price by the discount percentage, then subtract that amount from the original price. Alternatively, multiply the original price by (1 minus the discount percentage).
When multiple discounts are applied sequentially (like 20% off plus an extra 10% off), they do not add up to 30%. Instead, the second discount applies to the already-reduced price. A $100 item at 20% off becomes $80, then 10% off $80 is $8, giving you $72. That is effectively a 28% total discount, not 30%.
Retailers frequently inflate "original" prices to make discounts look larger. A useful strategy: ignore the percentage and focus on the final price. Ask yourself if you would buy the item at the sale price if there were no "was" price shown. If the answer is no, the discount is not saving you money because you would not have bought it otherwise.
In most U.S. states, sales tax is calculated on the discounted price, not the original price. So a $100 item at 30% off with 7% sales tax costs $70 x 1.07 = $74.90, not $100 x 1.07 x 0.70 = $74.90 (same result in this case, but the order of operations matters with coupons that have different tax treatments).