$E=k\frac{\left| q \right|}{{{r}^{2}}}\xrightarrow[{}]{(q)\,\,\,Fixed}\frac{{{E}'}}{E}={{(\frac{r}{{{r}'}})}^{2}}\Rightarrow \frac{160}{250}={{(\frac{{{r}_{0}}}{{{r}_{0}}+10})}^{2}}\Rightarrow \frac{4}{5}=\frac{{{r}_{0}}}{{{r}_{0}}+10}\Rightarrow 4{{r}_{0}}+40=5{{r}_{0}}\Rightarrow {{r}_{0}}=40cm$