Q. Half-life of a substance is $10\ years$. In what time, it becomes $\frac{1}{4}th$ part of the initial amount
A) $5\ years$
B) $10\ years$
C) $20\ years$
D) None of these
Explanation
(c) $\frac{N}{N_{0}} = \left( \frac{1}{2} \right)^{\frac{t}{t_{1/2}}} \Rightarrow \frac{1}{4} = \left( \frac{1}{2} \right)^{\frac{t}{10}}$
$\Rightarrow \frac{t}{10} = 2 \Rightarrow t = 20$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.