Q 12-13-010JEE MainAIEEE 2011Medium
The half life of a radioactive substance is $20$ minutes. The approximate time interval $(t_2 - t_1)$ between the time $t_2$ when $\dfrac{2}{3}$ of it has decayed and time $t_1$ when $\dfrac{1}{3}$ of it had decayed is
Answer: (B) $20$ min
At $t_1$, $\dfrac{2}{3}$ remains; at $t_2$, $\dfrac{1}{3}$ remains. The amount has halved between them, which takes exactly one half-life: $20$ min.
Solution by Sreeraj P, M.Sc Physics