A) 30 minutes
B) 40 minutes
C) 50 minutes
D) 38 minutes
E) 45 minutes
Correct Answer: B
Solution :
In first phase, \[K=\frac{2.303}{20}\log \frac{90}{100}\] .....(i) In second phase \[K=\frac{2.303}{t}\log \frac{81}{100}\] .....(ii) From eq. (i) \[\frac{2.303}{20}\log \frac{90}{100}=\frac{2.303}{t}\log \frac{81}{100}\] \[t=\frac{20(\log 81-\log 100)}{(\log 90-\log 100)}\] \[=\frac{20(1.908-2)}{(1.954-2)}=\frac{20\times (-0.092)}{(-0.046)}\]= 40 minutes
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