Question

You are buying potato chips at the grocery store. You have a manufacturer's coupon for $1.00 off and the store is offering 20% off if you buy 2 bags. How do you compute your final cost?

Answer 1

Let P be the regular price of one bag, r = 0.20 the store discount, and C the dollar value of the manufacturer coupon (usually C = 1.00; if the coupon is "per bag" and you use two, C = 2.00).

Two common sequences:

1) Store percent applied first, then coupon:
Final = 2P(1 − r) − C

2) Coupon applied first, then percent:
Final = (2P − C)(1 − r)

These differ by C·r (sequence 1 is cheaper by C·r).

Example (P = $3, C = $1, r = 0.20):
- Percent first then coupon: 2·3·0.8 − 1 = 4.80 − 1 = $3.80
- Coupon first then percent: (6 − 1)·0.8 = 5·0.8 = $4.00

So compute using the formula that matches how your store applies coupons (and check whether the manufacturer coupon is per bag).