a) 20 hours
b) 25 hours
c) 35 hours
d) Cannot be determined
Answer: c
Explanation: Suppose pipe A alone takes x hours to fill the tank.
Then, pipes B and C will take $$\frac{x}{2}$$ and $$\frac{x}{4}$$ hours respectively to fill the tank.
$$\eqalign{
& \frac{1}{x} + \frac{2}{x} + \frac{4}{x} = \frac{1}{5} \cr
& \frac{7}{x} = \frac{1}{5} \cr
& x = 35\,{\text{hours}} \cr} $$
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