Let x hours be the time taken by 1st pipe alone to fill the tank
then time taken by 2nd & 3rd pipe will be (x-5) & (x-9) hrs respectively
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)
x^2 - 18x + 45 = 0
(x-15) (x-3) = 0
(neglecting x=3) we have x =15