UPSC Prelims 2001·GS1·science-and-technology·science and technology

A radioactive substance has a half-life of four months. Three fourth of the substance would decay in

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Last updated 23 May 2026, 3:31 pm IST
  1. A3 months
  2. B4 months
  3. C8 monthsCorrect
  4. D12 months

Explanation

Half life is the time required for a substance to reduce to half of its initial quantity. At the start, you have 100 percent of the substance. After the first half life of 4 months, half of the substance decays, leaving 50 percent. After the second half life of another 4 months, half of the remaining 50 percent decays, which is 25 percent. This leaves 25 percent of the original substance. If 25 percent remains, it means 75 percent or three fourths of the substance has decayed. Therefore, it takes two half lives to reach three fourths decay. Since each half life is 4 months, two half lives equal 8 months.
science-and-technology: A radioactive substance has a half-life of four months. Three fourth of the substance would decay in
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