Q. The half life period of a radioactive substance is $5\ min$. The amount of substance decayed in $20\ min$ will be
A) 93.75%
B) 75%
C) 25%
D) 6.25%
Explanation
(a) Fraction of material decayed $= 1 - \frac{N}{N_{0}}$
$= 1 - \left( \frac{1}{2} \right)^{t/T_{1/2}} = 1 - \left( \frac{1}{2} \right)^{20/5} = 1 - \frac{1}{16} = \frac{15}{16} = 93.75\%$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.