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]