Answer
The magnitude of the force of repulsion is $16 F$
Work Step by Step
We can find an expression for the original electric force:
$F = \frac{k~q_1~q_2}{r^2}$
We can find an expression for the new electric force:
$F' = \frac{k~q_1~q_2}{(0.25~r)^2}$
$F' = 16\times \frac{k~q_1~q_2}{r^2}$
$F' = 16~F$
The magnitude of the force of repulsion is $16 F$.
