The electrostatic force between two charges is inversely
proportional to the square of the distance between them.
So if you want to multiply the force by, say, ' Q ',
you need to multiply the distance by ( 1 / √Q ) .
We want to multiply the force by 16, so we need to
multiply the distance by ( 1 / √16 ) = ( 1 / 4 ) .
The distance should be changed to 1/4 of what it is now.
