A radioactive substance has a half life of 20 days. What fraction of the original radioactive nuclei will remain after 80 days?

Answer Details

The half-life of a radioactive substance is the amount of time it takes for half of the original radioactive nuclei to decay. If the half-life of the substance is 20 days, then after 20 days, half of the original radioactive nuclei will have decayed, and half will remain. After another 20 days (a total of 40 days), half of the remaining nuclei will have decayed, leaving a quarter of the original nuclei. After another 20 days (a total of 60 days), half of the remaining nuclei will have decayed again, leaving an eighth of the original nuclei. Finally, after another 20 days (a total of 80 days), half of the remaining nuclei will have decayed, leaving a sixteenth of the original nuclei. Therefore, after 80 days, 1/16 of the original radioactive nuclei will remain. So the answer is (14).