Question

Two identical charges repel each other with a force 0.1mN and are moved additional 5.0cm a part the repulsive force is reduced to 25 x 10^-6N, (a) How far were they originally (b) what is the size of the charges

Answer 1

Is physics your School Subject?

Answer 2

electrical force (F) is proportional to electric field (E) and E is proportional to inverse square of the distance (1/r^2) between charges.

Therefore, F ~ 1/r^2.
Now compare the two force given F1 and F2 then

F1/F2 = r2^2 / r1^2