Question

Certain radioactive substance reduces to $25$% of its value in $16$ days. Its half-life is

• A. $32$ days
• B. $8$ days
• C. $64$ days
• D. $28$ day

Answer 1

The correct option is B $8$ days

Reduced to $25$% implies.
It is $\frac{1}{4}$ the initial value.
After '$n$' half lives, the substance would be $\frac{1}{2^n}$ of its initial.
i.e.
If $n=2$,
The fraction of the substance remaining would be $\frac{1}{2^2}=\frac{1}{4}$ or $25$%
This implies that $16$ days is two half lives.
Or that one half life is $8$ days.