An 800 g sample of a radioactive isotope has a half-life of 15.0 minutes. How much of the original sample remains after 45.0 minutes?

1. 45.0 minutes divided by a 15.0 minute half-life is exactly 3 half-lives. 2. After one half-life, 800 g becomes 400 g. 3. After a second half-life, 400 g becomes 200 g. 4. After a third half-life, 200 g becomes 100 g. 5. 50 g would need a fourth half-life, and 200 g and 400 g are the amounts left after only two or one half-lives. 6. So 100 g is correct. _Source: OpenStax Physics (CC BY 4.0), Ch 22 "The Atom", section 22.3 Half Life and Radiometric Dating_