Answer
0.03125 mg
Work Step by Step
Number of half-lives underwent= $\frac{335\,h}{67.0\,h}=5$
There are exactly five half-lives.
This means that the initial quantity is reduced by half five times.
Amount of sample left=
$1.000\,mg\times\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}\times\frac{1}{2}$
$=1.000\,mg\times\frac{1}{32}=0.03125\,mg$
