Discount calculations use 3 core formulas (2025 retail math): (1) **Discount Amount = Original Price × Discount %**.

Example: $200 jacket at 30% off → $200 × 0.30 = $60 discount. (2) **Final Price = Original Price - Discount Amount** OR **Final Price = Original Price × (1 - Discount %)**.

Example: $200 × (1 - 0.30) = $200 × 0.70 = $140 final price. (3) **Discount % = (Original Price - Sale Price) / Original Price × 100**.

Example: $200 original, $140 sale → ($200 - $140) / $200 × 100 = 30% discount.

**Multiple discounts (stacking)**: Apply sequentially, NOT additively.

Example: 20% off + additional 15% off ≠ 35% total.

Correct: $200 × 0.80 (after 20% off) = $160, then $160 × 0.85 (after 15% off) = **$136 final price** = 32% total discount (not 35%).

Formula: Final Price = Original × (1 - Discount1) × (1 - Discount2). **Common mistake**: Adding percentages directly. 20% + 15% = 35% is WRONG for sequential discounts.

Real saving: $200 - $136 = $64 = 32%, not $200 × 0.35 = $70.

**Reverse calculation (find original price)**: Original Price = Sale Price / (1 - Discount %).

Example: $140 sale price at 30% off → $140 / 0.70 = $200 original. **Business use**: Retailers use discount formulas to maintain profit margins.

Example: 40% markup + 25% discount = $100 cost × 1.40 markup = $140 retail, then $140 × 0.75 (after 25% off) = $105 sale price = 5% profit margin still preserved.

Black Friday 2025: Average discounts 20-40% for electronics, 30-50% for apparel, 10-25% for appliances.