The formula for the gravitational force has (distance)² in the denominator. That tells us that more distance produces less force, and that the force shrinks as fast as the square of the distance grows. Double the distance ==> 1/(2)² as much force = 1/4 the force = 4 units
Suppose that two objects attract each other with a gravitational force of 16 units. If the distance between the two objects is doubled, what is the new force if attraction between the two objects?