$1.50 / 25 = $.06 per bag
$3.00 / 75 = $.04 per bag
$3.75 / 125 = $.03 per bag
The best buy is the 125-bag package. Each bag costs 3 cents.

Gasoline example:
The price of the gasoline by paying with a credit card is: $17.10 (45 cents x 38 liters = $17.10. No cash discount)

Answers to extra problems:

Rope example:
$7.50 (3 yards x $2.50 = $7.50)

Dog food example:
The 10-pound bag at 69 cents per pound is the best buy: ($6.90 / 10 =$.69 per pound; $4.40 / 4 = $1.10 per pound)

Gasoline discount example:
If you use a credit card, you should pay $20.85.

(15 gallons x $1.39 = $20.85)
If you use cash, you should pay $20.25.
(15 gallons x $.04 = $.60 discount)
($20.85 - $.60 = $20.25)
Oil delivery example:
The seller's name and address; your name and address; the delivery date; the amount sold (350); the type of fuel (home heating oil), and the unit price.