A) 15 min
B) half an hour
C) one hour
D) one day
Correct Answer: B
Solution :
Let the initial quantity of the radioactive substance is 1 g. \[N={{N}_{0}}{{\left( \frac{1}{2} \right)}^{n}}\] \[\Rightarrow \] \[\left( 1-\frac{3}{4} \right)=1{{\left( \frac{1}{2} \right)}^{n}}\] \[{{\left( \frac{1}{2} \right)}^{2}}={{\left( \frac{1}{2} \right)}^{n}}\] \[\therefore \] \[n=2\] \[\Rightarrow \] \[T=n\times {{t}_{1/2}}\] \[60=2\times {{t}_{1/2}}\] \[{{t}_{1/2}}=30\]min or half an hour.
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