Afixed system of charges exerts a force of magnitude 18 n on a 9.0 c charge. the 9.0 c charge is replaced with a 4.0 c charge. what is the exact magnitude of the force (in n) exerted by the system of charges on the 4.0 c charge? do not include units with your answer.

According to the law of gravitational force:
the force between any two objects is directly proportional to the masses of the objects and inversely proportional to the square of the distance between these two objects.

Based on this:
if a 9 c charge is replaced with a 4 c charge while the distance between the charges is kept constant:
18/9 = F/4
2 = F/4
F = 2 x 4 = 8