Question

A radioisotope has half life of 10 years. What percentage of the original amount of it would you accept to remain after 20 years ?

• A. 0
• B. 12.5
• C. 25
• D. 8

Answer 1

Answer: (C)

25

$t_1/2$ = 10 years; T = 20 year
We know, T = $n{t}_{1/2}\Rightarrow n=\frac{20}{10}=2$
And, $N={\left(\frac{1}{2}\right)}^n{N}_0$
$\frac{N}{N_0}={\left(\frac{1}{2}\right)}^2=\frac{1}{4}$
$\%of\frac{N}{N_0}=\frac{1}{4}\times 100=25\%$