The half-life of a radioactive isotope is 10 days. What fraction of the initial amount of the isotope will remain after 30 days?
Correct: C
After one half-life (10 days), 1/2 of the isotope remains. After two half-lives (20 days), (1/2) * (1/2) = 1/4 remains. After three half-lives (30 days), (1/2) * (1/2) * (1/2) = 1/8 remains.