The amount of 217 mg of an isotope is given by A(t) = 217 € -0.0171, where t is time in years since the initial amount of 217 mg was present. Find the amount to the nearest milligraM left after 20 years

The amount left after 20 years = 154.15 mg

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 :

Then for t=20 years, the amount left :

answer; ///(u-235); ///is the correct