$$X(1+\frac{r}{100})^{2}-X=\frac{xxr\times4}{100}$$
$\left\{\begin{matrix}X=\frac{400x^{2}}{r+200}\text{, }&r\neq -200\\X\in \mathrm{C}\text{, }&r=0\text{ or }\left(x=0\text{ and }r=-200\right)\end{matrix}\right.$
$\left\{\begin{matrix}X=\frac{400x^{2}}{r+200}\text{, }&r\neq -200\\X\in \mathrm{R}\text{, }&r=0\text{ or }\left(x=0\text{ and }r=-200\right)\end{matrix}\right.$
$\left\{\begin{matrix}\\r=0\text{, }&\text{unconditionally}\\r=\frac{400x^{2}}{X}-200\text{, }&X\neq 0\\r\in \mathrm{C}\text{, }&X=0\text{ and }x=0\end{matrix}\right.$
$\left\{\begin{matrix}\\r=0\text{, }&\text{unconditionally}\\r=\frac{400x^{2}}{X}-200\text{, }&X\neq 0\\r\in \mathrm{R}\text{, }&X=0\text{ and }x=0\end{matrix}\right.$