A) 75 hrs
B) 65 hrs
C) 55 hrs
D) 45 hrs
Correct Answer: D
Solution :
Undecayed isotope \[=\frac{3.2\times {{10}^{6}}}{200\times {{10}^{6}}\times 1.6\times {{10}^{-19}}}=1\times {{10}^{17}}\] \[\therefore \frac{N}{{{N}_{0}}}={{\left( \frac{1}{2} \right)}^{t/T}}\Rightarrow \left( \frac{1}{8} \right)={{\left( \frac{1}{2} \right)}^{t/15}}\Rightarrow \frac{t}{15}=3\] or t =45 hours
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