Question

Half-life of a substance is 20 minutes. What is the time between 33% decay and 67% decay

• A. 40 minutes
• B. 20 minutes
• C. 30 minutes
• D. 25 minutes

Answer 1

Answer: (B)

20 minutes

Let $N_0$ be the number of nuclei at beginning
$\therefore$ Number of undecayed nuclei after $33\%$ decay = $0.67N_0$
and number of undecayed nuclei after $67\%$ of decay = $0.33N_0$
$\because$ $0.33N_0\simeq \frac{0.67N_0}{2}$ and in the half-life time the number of undecayed nuclei becomes half.