A) 50 minutes
B) 60 minutes
C) 70 minutes
D) 80 minutes
Correct Answer: B
Solution :
\[R=\frac{dN}{dt}\propto N\Rightarrow \frac{{{R}_{2}}}{{{R}_{1}}}=\frac{{{N}_{2}}}{{{N}_{1}}}\] But \[\frac{{{N}_{2}}}{{{N}_{1}}}={{\left( \frac{1}{2} \right)}^{{{t}_{1/2}}}}\Rightarrow \frac{25}{200}=\frac{1}{8}={{\left( \frac{1}{2} \right)}^{3}}\Rightarrow \frac{t}{{{t}_{1/2}}}=3\] \[\therefore {{t}_{1/2}}=\frac{t}{3}=\frac{3}{3}=1\ hour=60\]minutes
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