Answer
$3.09\times10^{3}$ years.
Work Step by Step
Decay constant $k=\frac{0.693}{t_{1/2}}=\frac{0.693}{5715\,y}=1.2126\times10^{-4}\,y^{-1}$
$\ln(\frac{A_{0}}{A})=kt$
$\implies \ln(\frac{27.5}{18.9})=0.375=1.2126\times10^{-4}\,y^{-1}(t)$
Then, $t=\frac{0.375}{1.2126\times10^{-4}\,y^{-1}}=3.09\times10^{3}\,y$
