Answer:
In this question the half-life (t½) of the isotope is 20 years, which means that after 20 years half of the isotope would have decayed and half would be left.
Since half of the sample remains, we can say that 10g/2 = 5g
After 20 years, 5 grams of the sample would remain.
If you wanted to know the amount after two half-lives, then after 40 years 2.5 grams of the isotope would exist.
