Question

The population of a town increases by 5% annually. If its population in 1981 was 138915, in 1978 it was:

Solution

$\mathrm{Population}\mathrm{in}1978=\frac{138915}{{\left(1+{\displaystyle \frac{5}{100}}\right)}^{3}}\phantom{\rule{0ex}{0ex}}=\left(138915\times \frac{20}{21}\times \frac{20}{21}\times \frac{20}{21}\right)\phantom{\rule{0ex}{0ex}}\mathbf{=}\mathbf{}\mathbf{1}\mathbf{,}\mathbf{20}\mathbf{,}\mathbf{000}$