How Stacked Discounts Work: Why 30% + 10% Is Not 40% Off
"30% off, plus an extra 10% at checkout" sounds like 40% off. It is not, because the second discount is taken from the already reduced price.
The formula
Final price = original × (1 − first %) × (1 − second %).
Total discount = 1 − (1 − first %) × (1 − second %).
Examples
- $20, 30% then 10%: $20 × 0.70 = $14.00, then × 0.90 = $12.60. That is 37% off, not 40%.
- $50, 50% then 20%: $25.00, then $20.00. 60% off, not 70%.
- Order does not matter: 10% then 30% gives the same $12.60.
Coupons in fixed amounts
A $5 coupon is different: apply it where the store applies it. $5 off before a 20% discount saves more than $5 off after it, so check the receipt order.
Doing it in the app
The Aislemath Discount tab has Discount and Extra discount fields. Enter both and it shows the final price, the price each for your quantity, how much you save and the total percentage off, then one tap adds it to your cart.