Question

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. $30$

Answer 1

Answer: (B)

decay $40\% \rightarrow 85\%$
Remaining $60\% \rightarrow 15\%$
$60\%\overset{t_{1 / 2}}{ \rightarrow }30\%\overset{t_{1 / 2}}{ \rightarrow }15\%$
$\therefore t=2t^{1 / 2}=60min$