The half life of a radioactive substance is 20 minutes. The time taken between 50% decay and 87.5% decay of the substance will be

$$\begin{gathered} 50\% \mathrm{decay} \mathrm{is} \mathrm{half} \mathrm{life} =20 \min \\ \mathrm{Next} 25\% \mathrm{decays} \mathrm{again} \mathrm{in} 20 \min \\ \mathrm{Further} 12.5 \mathrm{decays} \mathrm{in} 20 \min \\ \mathrm{Further} 12.5 \mathrm{decays} \mathrm{in} 20 \min \\ \mathrm{So} \mathrm{difference} \mathrm{in} \mathrm{time} \mathrm{of} 87.5\% \mathrm{decay} \mathrm{and} 50\% \mathrm{decay} \mathrm{is} =60-20=40\min . \end{gathered}$$