با توجه به قانون کولن و اصل پایستگی بار داریم: $\left. \begin{matrix}   F=k\frac{\left| {{q}_{1}} \right|\left| {{q}_{2}} \right|}{{{r}^{2}}}  \\   {F}'=k\frac{\left| {{{{q}'}}_{1}} \right|\left| {{{{q}'}}_{2}} \right|}{{{{{r}'}}^{2}}}  \\\end{matrix} \right\}\Rightarrow \frac{{{F}'}}{F}=\frac{\left| {{{{q}'}}_{1}} \right|\left| {{{{q}'}}_{2}} \right|}{\left| {{q}_{1}} \right|\left| {{q}_{2}} \right|}\times {{(\frac{r}{{{r}'}})}^{2}}\to {{{q}'}_{1}}={{{q}'}_{2}}=\frac{{{q}_{1}}+{{q}_{2}}}{2}=\frac{3+(-8)}{2}-2/5\mu C$ $\frac{{{F}'}}{F}=\frac{\left| -2/5 \right|\left| -2/5 \right|}{\left| +3 \right|\left| -8 \right|}\times {{(\frac{12}{10})}^{2}}\Rightarrow \frac{{{F}'}}{F}=\frac{3}{8}$