Chapter 19 - Sections 19.1-19.12 - Exercises - Problems by Topic - Page 946: 50

23.4 days

$R_{0}=5.88\times10^{4}/s$ $R=287/s$ $k=\frac{0.693}{t_{1/2}}=\frac{0.693}{3.042\,d}=0.2278/d$ Recall that $\ln(\frac{R}{R_{0}})=-kt$ $\implies t=-\frac{\ln(\frac{R}{R_{0}})}{k}=-\frac{\ln(\frac{287}{5.88\times10^{4}})}{0.2278/d}=23.4\,d$