At a certain time, 50% of a radioactive sample remains undecayed. After 10 seconds, only 12.5% of the sample remains. The half-life of the substance is:
Correct answer: C. 5 s
- A. 2.5 s
- B. 3.3 s
- C. 5 s
- D. 10 s
Explanation
Going from 50 percent to 12.5 percent halves the sample twice, from 50 to 25 to 12.5, so two half lives take 10 seconds and one half life is 5 seconds. Treating it as one half life gives 10 seconds, the trap answer.
This question appeared on the UHS MDCAT 2026 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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