Answer with Explanation:

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.

Therefore, 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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