Correct Answer: Option C
Suppose, first pipe alone takes x hours to fill the tank .
Then, second and third pipes will take (x -5) and (x - 9) hours respectively to fill the tank.
So, 1/x + 1/(x - 5) = 1/(x - 9)
=> (x - 5 + x) / x(x - 5) = 1/(x – 9)
=> (2x - 5)(x - 9) = x(x - 5)
=> x2 - 18x + 45 = 0
=> (x - 15)(x - 3) = 0
=> x = 15. [neglecting x = 3]
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