A) 40 yr
B) 60 yr
C) 80 yr
D) 100 yr
Correct Answer: B
Solution :
As \[\frac{N}{{{N}_{0}}}={{\left( \frac{1}{2} \right)}^{n}}\] \[\frac{N}{{{N}_{0}}}={{\left( \frac{1}{2} \right)}^{3}}=\frac{1}{8}\] Number of half lives = 3 \[\Rightarrow \] \[T=20yr\] \[\therefore \] \[T=\frac{t}{n}\] or \[t=T\times n=20\times 3yr=60yr\]
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