Question

A crystal contains a radioactive isotope that has a half-life of 20,000 years. You analyze a sample and find that one-eighth of the isotope remains (12.5%). How old is the sample? How many half-lives have passed?

Answer 1

Every 20,000 years, the amount is halved.
If you see that it is one eighth, that means it halved three times.
So, three half lives passed.
One half life is 20,000 years.
Three half lives is 60,000 years.