If the rate of increase in population is $r\%$ per year and the population after $n$ years is $p$, then the population $n$ years before was:

Let the population $n$ years before be $P_0$.
The population increases at a uniform rate of $r\%$ per year. Therefore, after $n$ years, the final population $p$ is given by:

$$p=P_0{\left(1+\frac{r}{100}\right)}^n$$

To express the population $n$ years before ($P_0$) in terms of $p$, we isolate $P_0$:

$$P_0=\frac{p}{{\left(1+\frac{r}{100}\right)}^n}$$

$$P_0=p{\left(1+\frac{r}{100}\right)}^{-n}$$