Question

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

• A. 20 min
• B. 30 min
• C. 40 min
• D. 25 min
• E. 10 min

Answer 1

The correct option is C 40 min

For $50\%$ decay, time is equal to 1 half-life.
For $87.5$% decay, time is 3 half lives.
$\therefore$ Time interval $\Delta t=$ 2 half-lives.
$\Delta t=2\times 20min$ $=40min$