The force of gravity is proportional to 1/r^2, where r is the radius (distance) between the objects.
So if the force was originally[tex] \frac{1}{r^2} [/tex], doubling the distance between them would make the new force [tex] \frac{1}{(2r)^2} = \frac{1}{4r^2} = (\frac{1}{4}) \frac{1}{r^2} [/tex]
