The half-life of a radioactive substance is 30 minutes. The time taken between $40 \%$ decay and $85 \%$…
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
15 minutes
90 minutes
60 minutes
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.