Percentage off calculations are among the most commonly needed math in everyday life, especially during sales and shopping. The formula is straightforward: Discount Amount = Original Price × (Discount Percentage ÷ 100). The sale price is then: Sale Price = Original Price - Discount Amount.
Example: A $80 item at 25% off has a discount of $80 × 0.25 = $20. The sale price is $80 - $20 = $60. With 8% sales tax, the final price is $60 × 1.08 = $64.80.
Stacking discounts: Some stores offer "extra 20% off sale items." These are typically applied sequentially, not added together. A $100 item marked 30% off is $70. An additional 20% off reduces it to $70 × 0.80 = $56 — not $100 × 0.50 = $50. Sequential discounts always result in a higher final price than adding the percentages.
Quick mental shortcuts: For 10% off, move the decimal one place left ($45 → $4.50 discount). For 20%, double the 10% amount. For 25%, divide by 4. For 50%, divide by 2. For 15%, calculate 10% and add half of that.