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Question: Answered & Verified by Expert
The half-life of a radioactive substance is 30 minutes. The time taken between $40 \%$ decay and $85 \%$ decay of the same radioactive substance is
PhysicsNuclear PhysicsJEE Main
Options:
  • A 15 minutes
  • B 90 minutes
  • C 60 minutes
  • D 30 minutes
Solution:
2524 Upvotes Verified Answer
The correct answer is: 60 minutes
When $40 \%$ nuclei decay, $60 \%$ nuclei remain undecayed Let this number be $\mathrm{N}_0$ When $85 \%$ nuclei decay, $15 \%$ nuclei remain undecayed This number will be $\mathrm{N}=\frac{\mathrm{N}_0}{4}$
The number of nuclei will become one-fourth in two half-lives i.e., 60 minutes.

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