A) 40 min
B) 20 min
C) 25 min
D) 30 min
Correct Answer: A
Solution :
\[\frac{20}{80}=\frac{1}{{{2}^{t/20}}}\,\,or\,\,\frac{1}{4}=\frac{1}{{{2}^{t/20}}}\,\,or\,\,\frac{1}{{{2}^{2}}}=\frac{1}{{{2}^{t/20}}}\] \[or\,\,\,\,\,\,\frac{t}{20}=2\,\,\,or\,\,t=40\,\,\min \]
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