Answer:
36 hours.
Step-by-step explanation:
Let x represent time taken by both pipes to fill the tank.
We have been given that an inlet pipe can fill a swimming pool in 9 hours. So part of pool filled by inlet pipe in one hour would be
.
We are also told that an outlet pipe can empty the pool in 12 hours. The part of pool emptied by outlet pipe in one hour would be
.
Sine the time taken by both pipes to fill the tank is x hours, so part of pool filled by both pipes in one hour would be
.
We will get an equation using our given information as:

Let us solve for x.




Cross multiply:



Therefore, it will take 36 hours for both pipes to fill the tank.