A) $ 5\frac{7}{11}h $
B) $ 4\frac{7}{11}h $
C) $ 8\frac{7}{11}h $
D) $ 3\frac{7}{11}h $
E) None of these
Correct Answer: D
$ \therefore $ Required time to fill the tank $ =\frac{40}{11}h=3\frac{7}{11}h $ Alternative method Here, $ m=8, $ $ n=10 $ and $ p=20 $
$ \therefore $ Required time to fill the tank $ =\frac{mp}{mn+np+mp} $ $ =\frac{8\times 10\times 20}{8\times 10+10\times 20+8\times 20} $ $ =\frac{1600}{80+200+160}=\frac{1600}{440} $ $ =\frac{40}{11}=3\frac{7}{11}h $