What is the correct formula for figuring out mass 1 from mass 2 and distances 1 and 2 (distance 1 is the distance from the center to mass 1 and distance 2 is the distance from the center to mass 2)?

Automobile fuel efficiency is often measured in miles that the car can be driven per gallon of fuel (mpg). heavier cars tend to have lower fuel efficiencies (lower mpg ratings). which variable (weight or fuel efficiency) is most naturally considered the explanatory (predictor) variable in this context?