Two charges attract each other with a force of 3.0 n. what will be the force if the distance between them is reduced to one-ninth of its original value?

The electrostatic force between two charges is given by

where ke is the Coulomb's constant, q1 and q2 are the two charges and d is the distance between the two charges.
We can see from the formula that the force is proportional to . This means that if we reduce the distance to one-ninth, i.e. the new distance is

then the force will scale as
 
So, the new force will be 81 times stronger than the initial value, therefore the new force is