The half life of #"^131I# is 8.021 days. What fraction of a sample of #"^131I# remains after 24.063 days?
After three half-lines (
Start with your sample of iodine-131. In one half-life, just over eight days, half of the sample decays leaving the other half. That remaining half is in effect its own sample in which half decays again after another half-life. You're now left with one-fourth of the original iodine-131 with half decaying in the third half-life from when you started.