Question

A radioactive isotope has a-life of 10 yr. What percentage of the original amount of it ramain after 20 yr?

• A. 0
• B. 12.5
• C. 8
• D. 25

Answer 1

The correct option is D 25

solution:

$t_{\frac{1}{2}}=10years,T=20years$

We know$T=n{t}_{\frac{1}{2}}\Rightarrow n=\frac{20}{10}=2$

And $N=(\frac{1}{2}{)}^n{N}_o$

$\frac{N}{N_o}=(\frac{1}{2}{)}^2=\frac{1}{4}$

%$of\frac{N}{N_o}=\frac{1}{4}\times 100=25$%

hence the correct opt: D