Answer
$Q(t)\approx Q_{0}e^{-0.0248t}$
Work Step by Step
$Q(t)=Q_{0}e^{-kt}$
The decay constant k and half-life $t_{h}$ are related by
$t_{h}k=\ln 2$
$28k=\ln 2$
$k=\displaystyle \frac{\ln 2}{28}\approx 0.0248$
$Q(t)\approx Q_{0}e^{-0.0248t}$