The half-life is 47 days, which means that for any starting amount [tex]a_0[/tex] of Hg-203, we're left with [tex]\dfrac{a_0}2[/tex] after this amount of time.
[tex]\dfrac{a_0}2=a_2e^{47k}\implies\dfrac12=e^{47k}\implies-\dfrac{\ln2}{47}=k[/tex]
So, the amount of Hg-203 [tex]a(d)[/tex] left after [tex]d[/tex] days is given by
[tex]a(d)=a_0e^{-(\ln2)/47\,d}[/tex]
2 weeks = 14 days, so if we start with 400 mg, after two weeks we end up with
[tex]a(14)=400e^{-(\ln2)/47\cdot14}\approx325.38[/tex] mg
