Answer:
3.6 hours.
Step-by-step explanation:
We work in terms of rates per hour:
1/9 + 1/6 = 1/x where x is the time for 2 pipes working together.
Multiply through by the LCD of 6, 9 and x ( which is 18x) :-
2x + 3x = 18
5x = 18
x = 3.6 hours.
