$$A = P r + \frac { P ( n ) } { 100 }$$
$A=\frac{P\left(n+100r\right)}{100}$
$\left\{\begin{matrix}P=\frac{100A}{n+100r}\text{, }&r\neq -\frac{n}{100}\\P\in \mathrm{R}\text{, }&A=0\text{ and }r=-\frac{n}{100}\end{matrix}\right.$
