Question

The half-life period of a radioactive material is $15\, \min.$ What per cent of radioactivity of that material will remain after $45$ minutes?

• A. 0.175
• B. 0.15
• C. 0.125
• D. 0.1

Answer 1

Answer: (C)

0.125

Answer 2

$T=45\, \min , t_{1 / 2}=15\, \min$ $\because T=n \times t_{1 / 2}$ $\Rightarrow n=\frac{45}{15}=3$ $\Rightarrow n=3$ $N=N_{0} \times\left(\frac{1}{2}\right)^{3}$ Let $N_{0}=1$ $N=1 \times \frac{1}{8}, N=\frac{1}{8}$ $N=\frac{1}{8} \times 100=12.5 \%$ $N=12.5 \%$