You can set up a simple equation based on the information in the question:
[tex]m=m_0 ( \frac{1}{2} )^n[/tex]
Where m is the final mass, m0 is the initial mass, and n is the number of half-lives that have passed. We're told in the question that m0=4000 and n=8. Subbing these numbers in and solve for m gives 15.625 mg.
