The amount left after 20 years = 154.15 mg
Further explanation
The atomic nucleus can experience decay into 2 particles or more due to the instability of its atomic nucleus.
Usually radioactive elements have an unstable atomic nucleus.
The main particles are emitted by radioactive elements so that they generally decay are alpha (α), beta (β) and gamma (γ) particles
The decay formula for isotope :
[tex]\tt \large{\boxed{\bold{A(t)=217e^{-0.0171t}}}[/tex]
Then for t=20 years, the amount left :
[tex]\tt A(t)=217e^{-0.0171\times 20}\\\\A(t)=154.15~mg[/tex]
