A) 20 days
B) 40 days
C) 60 days
D) 80 days
Correct Answer: B
Solution :
\[{{T}_{1/2}}=20\,\,days\] \[N={{N}_{0}}-\frac{3}{4}\,\,\,{{N}_{0}}=\frac{{{N}_{0}}}{4}\] \[N={{N}_{0}}{{\left( \frac{1}{2} \right)}^{n}}\,\,\,\,\,\,\Rightarrow \,\,\,\,\,\frac{1}{4}={{\left( \frac{1}{2} \right)}^{n}}\,\,\Rightarrow \,\,n=2\] \[\therefore \,\,\, Time taken = 2 half-lives\] \[= 2 \times 20 = 40 day\]
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