A gallon of milk sells for $3.09 and a quart of milk sells for $0.80.

A gallon of milk sells for $3.09 and a quart of milk sells for $0.80.

Let's get this in terms of a common measurement for unit cost. Let's use gallons:
1 gallon = 4 quarts

Gallon of milk
$3.09 / gallon

Quart of milk
0.80 / 0.25 gallons

Multiply top and bottom by 4:
$3.20 / gallon

This means the gallon is the better buy