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NDA 2021 GAT · General Knowledge · Nuclear and Modern Physics · medium

The half-life of a radioactive substance is 10 days. The fraction of the original amount remaining after 30 days is

Single correct option

The half-life of a radioactive substance is 10 days. The fraction of the original amount remaining after 30 days is
  1. A. 1/3
  2. B. 1/8
  3. C. 1/6
  4. D. 1/9
Show answer and solution

Answer: (B) 1/8

30 days is three half-lives. The amount left is (1/2)^3 = 1/8. Option 1/3 arises from dividing time linearly, but decay is exponential: each half-life halves the remaining quantity, giving 1/2, 1/4 and then 1/8.
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Original question written in the NDA 2021 GAT pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake