Answer: 50 mg
Step-by-step explanation:
Substituting into the half-life formula, we get that
[tex]N(t)=800\left(\frac{1}{2} \right)^{t/25}[/tex]
So, after 100 years,
[tex]N(100)=800\left(\frac{1}{2} \right)^{100/25}=\boxed{50 \text{ mg}}[/tex]
