Answer
Consider the force between 2 charged objects. Coulomb observed that if the charge on one of the objects were doubled, then the force would be doubled.
Work Step by Step
Consider the force between 2 charged objects. Coulomb observed that if the charge on one of the objects were doubled, then the force would be doubled. This agrees with a law that says that the force is proportional to the product of the charges, but is not consistent with saying that the force is proportional to the sum of the charges.
Furthermore, he observed that a charged object exerts no force on a neutral insulating object. Once again, this agrees with a law that says that the force is proportional to the product of the charges Q and 0, but is not consistent with saying that the force is proportional to the sum of the charges (for then there would be a force).