The correct answer is 35 days .
Let T days be the sample's half-life.
Therefore, after T days, 1 / 2 of the sample is still present, followed by 1 / 4 after 2T days, and 1 / 8 after 3T days.Consequently, three half-lives have ended in 21 days.3T also equals 21 days.T is therefore (21 / 3) days, or 7 days.
After 3T days, 1/8 of the sample is still alive; after 4T days, 1/16; and after 5T days, 1/32 is still alive.Consequently, 5T = 5 x 7 days = 35 days have passed since the first amount of radioactive nuclei, which was 1 / 32 of the total amount, was present.A radionuclide's half-life is the amount of time it takes for half of its radioactive atoms to decay.
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