The point charges Q and -2Q are placed at some distance apart. If the electric field at the location of Q is E, the . electric field at the location of -2Q will be 

$$\begin{gathered} \text{ Also, electric field at the location of Q }{\mathrm{E}}_1=\frac{1}{4{\mathrm{\pi \epsilon }}_0}\frac{(-2\mathrm{Q})}{{\mathrm{r}}^2} \\ \mathrm{it} \mathrm{is} \mathrm{due} \mathrm{to} -2\mathrm{Q} \mathrm{and} \mathrm{towards} -2\mathrm{Q} \mathrm{or} \mathrm{away} \mathrm{from} \mathrm{Q} \\ \mathrm{Electric} \mathrm{field} \mathrm{at} \mathrm{the} \mathrm{location} \mathrm{of} 2\mathrm{Q} \mathrm{it} \mathrm{is} \mathrm{due} \mathrm{to} \mathrm{Q} \mathrm{and} \mathrm{away} \mathrm{from} \mathrm{Q} \\ {\mathrm{E}}_2= \frac{1}{4{\mathrm{\pi \epsilon }}_0}\frac{\mathrm{Q}}{{\mathrm{r}}^2} \\ {E}_2 and E_1 both are in the same direction. \\ so \frac{E_1}{E_2} =2 \\ {E}_2=\frac{E}{2} \end{gathered}$$