$\begin{align}  & \left. \begin{matrix}   {{F}_{10}}=k\frac{\left| {{q}_{1}}{{q}_{0}} \right|}{{{d}^{2}}}=k\frac{2\times {{10}^{-18}}}{{{d}^{2}}}  \\   {{F}_{20}}=k\frac{\left| {{q}_{2}}{{q}_{0}} \right|}{{{(\frac{d}{2})}^{2}}}=k\frac{4\times {{10}^{-18}}}{{{d}^{2}}}  \\\end{matrix} \right\}\Rightarrow \frac{{{F}_{20}}}{{{F}_{10}}}=2 \\  & \left. \begin{matrix}   {{F}_{T}}=\sqrt{F_{10}^{2}+F_{20}^{2}}=\sqrt{F_{10}^{2}+4F_{10}^{2}}=\sqrt{5}{{F}_{10}}  \\   {{F}_{T}}=4\sqrt{5}\mu N\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\\end{matrix} \right\}\Rightarrow {{F}_{10}}=4\mu N \\ \end{align}$