An electricity and magnetism problem by Razing Thunder

Force between $2$ charges is given by $F=\dfrac{q_{1}q_{2}}{4\pi\epsilon_{0} r^2}$ , where $q_{1}$ and $q_{2}$ are charges and $r$ is the distance between the charges.

Now, if $q_{1}$ is made $1.1q_{1}$ and $q_{2}$ is made $0.9q_{2}$ , their product becomes $0.99q_{1}q_{2}$ .

Hence, the new force between them is $0.99$ times the original value which is $\boxed{9.9~N}$