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\(\frac{1}{r^2}\), doubling the distance between them would make the new force \(\frac{1}{(2r)^2} = \frac{1}{4r^2} = (\frac{1}{4}) \frac{1}{r^2}\)
