* Understanding the Relationship: You know that 1 gallon is equal to 4 quarts. This means there are 4 quarts for every 1 gallon.

* Setting up the Ratio: To convert quarts to gallons, you want the quarts to cancel out. This means you need a ratio with quarts in the denominator:

* 1 gallon / 4 quarts

* Multiplying: When you multiply 15 quarts by this ratio, the quarts units cancel out:

* 15 quarts * (1 gallon / 4 quarts) = 3.75 gallons

Therefore, the ratio you can multiply by to convert quarts to gallons is 1 gallon / 4 quarts.