Find the rate of interest for a sum of money

$ \text\frac{A}{P} $ = 1.728, n = 3  years compounded annually
Compound Interest formula:  A = P $ (1 + \frac{r}{100})^n $
$ (1 + \frac{r}{100})^3 = 1.728 $
$ 1 + \frac{r}{100} = \sqrt[3]{1.728} $
$ 1 + \frac{r}{100} = \frac{6}{5} $
$ \frac{r}{100} = \frac{6}{5} - 1 = \frac{1}{5} $
$ r = \frac{1}{5} \times 100 = 20% $
Answer: 20%