Force, Motion, Gravity and Equil Test

How do we get an equation saying that gravitational force is proportional to the inverse distance squared?
-----
The equation here is the universal gravitation equation:

F_g = (Gm₁m₂)/r²

This equation is definitely one to know by heart for the MCAT.  If we assume that the masses are constant, and G is always constant, we can reduce the equation to

F_g α 1/r²

This thus shows us that gravitational force is proportional to the inverse of the distance, squared.