Answer
$t = 65\ years$
Work Step by Step
Integrated first order rate law is given as:
$ln\frac{[A]_{t}}{[A]_{0}} = -\lambda t$
$\lambda = \frac{0.693}{t_{\frac{1}{2}}} = \frac{0.693}{28.1 years} = 0.0247 year^{-1}$
Hence, $ln\frac{0.2}{1} = - 0.0247 t$
or, $t = 65\ years$
