Where G is a constant, M is the mass of the larger object, m is the mass of the other object, and r is the distance between the two objects.
So:
1600 = GMm/r^2
Now we can find the force when the distance doubles:
F = GMm/(2r)^2
GMm will remain the same since G is a constant and the masses didn't change, so the only difference between this and the first calculation is the denominator.
By simplifying the denominator, we find:
F = GMm/(4r^2)
Now we can see that the only difference between this and the equation given the original distance is that we are dividing by 4. This means that the force will be one fourth of the original force:
