Answer
23.4 days.
Work Step by Step
Recall: $\ln \frac{A_{t}}{A_{0}}=-\frac{0.693}{t_{1/2}}t$
$\implies \ln \frac{287/s}{5.88\times10^{4}/s}=-\frac{0.693}{3.042\,d}\times t$
$\implies -5.322415=- 0.22781\,d^{-1}\times t $
$\implies t= \frac{-5.322415}{-0.22781\,d^{-1}}$
$= 23.4\,d$