(a) Two charged particles exert an electric force of 21 N on each other. What will the magnitude of the force be if the distance between the particles is reduced to one-fourth of the original separation?
Answer: 336 N

(b) If the original distance between the particles is now restored, how much larger would the charge on either one of the particles have to become to yield the same force that was present when the particle separation was only one-fourth its original value?
Answer: ?