Lightning-Fast Discount & Sale Calculations

Mental Math for Shopping

Calculate discounts and sale prices faster than reaching for your phone!

Basic Discount Formula

Sale Price = Original Price - Discount Amount

Or:

Sale Price = Original Price × (100% - Discount %)

Common Discounts

10% off:

• Divide by 10, then subtract
• $80 with 10% off: $80 - $8 = $72

20% off:

• Find 10%, double it, subtract
• $150 with 20% off: 10% = $15, 20% = $30, pay $120

25% off:

• Divide by 4, then subtract
• $200 with 25% off: $200 ÷ 4 = $50 off, pay $150

30% off:

• Find 10%, triple it, subtract
• $90 with 30% off: 10% = $9, 30% = $27, pay $63

50% off:

• Simply divide by 2
• $76 with 50% off: $38

The Complement Method

Instead of subtracting, multiply by what's left:

20% off = pay 80%

• $50 × 0.80 = $40

30% off = pay 70%

• $90 × 0.70 = $63

15% off = pay 85%

• Find 85%: $100 × 0.85 = $85

Multiple Discounts

When there are stacked discounts (rare but happens):

Example: 20% off, then additional 10% off

• Start: $100
• After 20% off: $80
• Then 10% off $80: $80 - $8 = $72
• NOT the same as 30% off ($70)!

Sales Tax Addition

After discount, add tax:

Example: $80 item, 25% off, 8% tax

1. Sale price: $80 × 0.75 = $60
2. Tax: $60 × 0.08 = $4.80
3. Total: $64.80

Quick Estimation

For rough estimates:

• 20% off ≈ pay 4/5 of price
• 25% off ≈ pay 3/4 of price
• 33% off ≈ pay 2/3 of price

Real Scenarios

Clearance rack: $45, 40% off

• 40% = 10% × 4
• 10% of $45 = $4.50
• 40% = $18
• Pay: $45 - $18 = $27

BOGO 50% (Buy One Get One 50% Off)

• Item: $30 each
• First: $30
• Second: $15 (50% off)
• Total: $45 for both (= 25% off total)