Q148 / 150 · MHT CET 2021 (24 Sep Shift 2) · Physics

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

  • A
    15 minutes
  • B
    90 minutes
  • Correct answer
    C
    60 minutes
  • D
    30 minutes

Solution

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.

Hence, the correct answer is option: 'C'.

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