Answer :
Take into account that the electric force between two charges q1 and q2, separated by a distance r, is given by:
[tex]F=K\frac{q_1q_2}{r^2}[/tex]where K is the Coulomb's constant.
If r is changed to 3r and q1 is changed to 4q1, the new force F' is given by:
[tex]\begin{gathered} F^{\prime}=K\frac{(4q_1)(q_2)}{(3r)^2}=\frac{4}{9}K\frac{q_1q_2}{r^2} \\ F^{\prime}=\frac{4}{9}F \end{gathered}[/tex]Hence, the factor of change of the electric force is 4/9