A radioactive sample has a half life of 8 days. The fraction remaining undecayed after 24 days is

Explanation

Twenty four days is three half lives, so the sample is halved three times, leaving one half multiplied by one half multiplied by one half, which is one eighth. A common error is to divide 24 by 8 and answer one third, which misunderstands the exponential nature of decay. Iodine 131, used in thyroid treatment, has almost exactly this half life.