# If a radioactive sample has half the radioactive atoms left after 20 years, what is its half-life?

##### 1 Answer

May 31, 2014

The answer is 20 years.

A half life is the time taken for the radioactive particles in a sample to decay by half. It would, therefore, take another 20 years for the half life to reach 1/4 of what it originally was.

The decay series by half lives looks like this: