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The half-life of a radioactive substance is 30 minutes. The time (in minutes) taken between 40% decay and 85% decay of the same radioactive substance is:
A
45
B
60
C
15
D
30
Explanation
60% → 15% is two half-lives.
Detailed Solution
Decay from 40% to 85% means remaining goes from 60% to 15%.
$60\% \xrightarrow{t_{1/2}} 30\% \xrightarrow{t_{1/2}} 15\%$
$\therefore t = 2t_{1/2} = 60$ min
$60\% \xrightarrow{t_{1/2}} 30\% \xrightarrow{t_{1/2}} 15\%$
$\therefore t = 2t_{1/2} = 60$ min