After 3 half-lives, the remaining fraction of a radioactive sample is:
Correct answer: C. 1/8
- A. 1/2
- B. 1/4
- C. 1/8
- D. 1/16
Explanation
Each half life halves the number of undecayed nuclei, so three of them give a half multiplied by a half multiplied by a half, which is one eighth. Decay is exponential, so the fractions multiply rather than the half lives adding in any simple way. After four half lives one sixteenth would remain.
This question appeared on the UHS MDCAT 2025 paper, which you can sit online with every answer explained.
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About Half-life and Rate of Decay
Half-life is the time required for half the unstable nuclei in a sample to decay, and it remains constant for a particular radioactive nuclide. Questions use the exponential decay law, decay constant, activity, remaining quantity and mean life. Radioactive decay is random for individual nuclei but predictable for large samples.
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