B. 15 hours
Feedback 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.
As per question, we get
1/x + 1/(x−5) = 1/ (x−9) => x−5+x/x(x−5) = 1/ x−9
=>(2x−5)(x−9)=x(x−5)
=>x^2−18x+45=0
After solving this euation, we get
(x-15)(x+3) = 0,
As value can not be negative, so x = 15