Quick discount math, including the stacked-discount trap
Calculating a single discount is straightforward: multiply the original price by the discount percentage to find your savings, then subtract that from the original price for the final cost.
Savings = Original price × (discount % ÷ 100)
Final price = Original price − Savings
Stacked discounts don't simply add
A common mistake is assuming two discounts — say, 20% off plus an extra 10% off — combine into a flat 30% off. They don't. Each discount applies to the already-reduced price from the previous step, so the combined effect is always slightly less than simply adding the percentages together. Calculating each step separately, using the previous result as the new starting price, gives the accurate final number.