Q. The half -life period of a radioactive substance is 3 days. Three fourth of substance decays in
A) 3 days
B) 6 days
C) 9 days
D) 12 days
Explanation
(b) $N = N_{0}\left( \frac{1}{2} \right)^{n}$
Remaining part $= \ N_{0}\ - \ \frac{3}{4}N_{0}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \frac{1}{4}N_{0}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{N_{0}}{4} = N_{0}\left( \frac{1}{2} \right)^{n}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \left( \frac{1}{2} \right)^{2} = \left( \frac{1}{2} \right)^{n}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ n$=2
Time = Half year $\times$ Number of half year= 3$\times$2 = 6days
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.