Radioactive iodine-131 decays with a half-life of 8.0 days. A chemist obtains a fresh sample of iodine-131 and measures its radioactivity.

Question

She then determines that to do an experiment, the radioactivity cannot fall below 25% of the initial measured value. How long does she have to perform the experiment?

Answer 1

After one half-life of 8 days, 100% drops to 50%. After another half-life, i.e. 16 days after starting, the 50% drops to 25%.