The amount left after 20 years = 154.15 mg
<h3>Further explanation
</h3>
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 :

Explanation:
Flow-duration data are daily mean flow values measured over a specified time interval that have been exceeded various percentages of the specified time interval.
The process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water.
13 atoms are in the chemical formula AI(PO4)3
hope that helps :-)