Question

The number of half lives elapsed before $93.75 \%$ of a radioactive sample has decayed is

• A. 6
• B. 4
• C. 2
• D. 8

Answer 1

Answer: (B)

Number of half lives be obtained by using
$N=N_{0}\left(\frac{1}{2}\right)^{n}$
Here, $N=(100-9375) \times \frac{N_{0}}{100}$
$=\frac{6.25}{100} \times N_{0}$
$\therefore \left(\frac{1}{2}\right)^{n}=\frac{625}{10000}$
$=\left(\frac{5}{10}\right)^{4}=\left(\frac{1}{2}\right)^{4}$
or $n=4$