Question 962204
Not solvable because the last sentence is incomplete:
"If the pool is 13 filled and then the inlet pipe and drain pipe are opened, "
You need to straighten that information.


Regardless, the fill rate for both pipes open at the same time will be  (1/12-1/36) pools per hour.  That is {{{3/36-1/36=2/36=1/18}}}.   That means, 1 pool's worth in 18 hours, filling rate.


What is, "pool is 13 filled"?    What is the capacity of this pool?  What did the "13" mean?