If [tex]y(t)[/tex] is the mass (in mg) remaining after [tex]t[/tex] years, then [tex]y(t) = y(0) (0.5)^{t/T} = 400 (0.5)^{t /4}[/tex], where [tex]T[/tex] is the half-life period and [tex]y(0)[/tex] is the amount at t = 0 years (initial).
Then at t = 20:
[tex]y(20) = 400 (0.5)^{20 /4} = \text{12.5 mg} [/tex]
