Answer
$2.66\times10^{3}\,y$
Work Step by Step
Recall: $\ln \frac{N_{t}}{N_{0}}=-\frac{0.693}{t_{1/2}}t$
$\implies \ln \frac{72.5}{100}=-\frac{0.693}{5730\,y}\times t$
$\implies -0.32158=-1.2094\times10^{-4}\,y^{-1}\times t $
$\implies t= \frac{-0.32158}{-1.2094\times10^{-4}\,y^{-1}}$
$= 2.66\times10^{3}\,y$