Question

The half of a radioactive substance is 20 s, the time taken for the sample to decay by 7/8^th of its initial value is.

• A. 20 s
• B. 40 s
• C. 60 s
• D. 80 s

Answer 1

Answer: (C)

60 s

Answer 2

: After three half lives, the fraction of un decayed nuclei

$= \left( \frac{1}{2} \right)^{3} = \frac{1}{8}$

$\therefore$Time taken for the sample to decay by $(1 - 1/8)^{th}$or $\frac{7^{th}}{8}$of initial value

$= 3T_{1/2} = 3 \times 20 = 60s$