If a source has a half-life of 20 years, how old would it be after 3 half-lives?

The age of a radioactive source can be calculated based on its half-life. The half-life is the time it takes for half of a radioactive substance to decay. If a source has a half-life of 20 years, after one half-life, it would be 20 years old. After two half-lives, it would accumulate to 40 years (20 years for the first half-life and another 20 years for the second). Finally, after three half-lives, you add another 20 years to the total, resulting in an age of 60 years.

This understanding highlights that the age of the radioactive source increases linearly with each half-life. Therefore, after three half-lives, the total age equates to 60 years.