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 :

Answer: The Milky Way is a barred spiral galaxy with a diameter between 150,000 and 200,000 light-years (ly).
These are buffer systems (solutions of substances) with a steady concentration of hydrogenium ions. At addition to such solution of a small amount of acid or alkali does not change рН.
For example:
CH₃COOH + CH₃COONa acetate buffer
KH₂PO₄ + K₂HPO₄ potassium phosphate buffer
Climate is an area's long-term weather patterns