We will investigate how to determine the mileage of any vehicle given the amount of distance travelled while burning a volume of fuel.
Susie's car teavells:

Susie's car cinsumes:

Mileage is a car performance parameter that defines the fuel economy. It is usually expressed as a ratio of distance travelled to the amount of fuel consumed as follows:

We will plug in the respective quantities in the above ratio as follows:

Simplfy:

Hence, Susie's car will travel 31 miles for each gallon of fuel consumed!