Answer
44.5 days
Work Step by Step
Recall that $\ln(\frac{R_{0}}{R})=kt=\frac{0.693}{t_{1/2}}t$
$\implies \ln(\frac{16800}{10860})=0.43629=\frac{0.693}{t_{1/2}}\times28.0\,d$
$\implies t_{1/2}=\frac{0.693\times28.0\,d}{0.43629}=44.5\,d$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.