We know that there are 32 ounces in a quart, so $0.89 per quart ÷ 32 ounces = $0.0278 per ounce.
One gallon is 128 ounces, when a one gallon size is purchased, the cost will be $3.20 ÷ 128 = $0.025 per ounce.
The cost difference is $0.0278 - 0.025 = $0.0028 per ounce.
3 gallons = 128 ounces x 3 = 384 ounces
When we multiply 384 ounces by $0.0028 per ounces, we get $1.0752, which is the savings.