بنابر رابطهٔ قانون کولن می‌توان نوشت: $\left\{ \begin{matrix}   {{F}_{12}}=\frac{k\left| {{q}_{1}} \right|\left| {{q}_{2}} \right|}{{{(\overline{AB})}^{2}}}  \\   {{F}_{23}}=\frac{k\left| {{q}_{2}} \right|\left| {{q}_{3}} \right|}{{{(\overline{BC})}^{2}}}  \\\end{matrix}\Rightarrow \frac{{{F}_{12}}}{{{F}_{23}}}=\frac{\left| {{q}_{1}} \right|\left| {{q}_{2}} \right|}{\left| {{q}_{2}} \right|\left| {{q}_{3}} \right|}\times {{(\frac{\overline{BC}}{\overline{AB}})}^{2}} \right.\Rightarrow 6=\frac{{{q}_{1}}}{2(3{{q}_{1}})}\times {{(\frac{\overline{BC}}{10})}^{2}}\Rightarrow \overline{BC}=60cm$