Question 33687
In 1 hour, you can fill 1/10 of the pool and in 1 hour you can empty 1/20 of the pool. If the two events occur simultaneously, then in 1 hour you can fill 1/10 - 1/20 = 1/20 of the pool.
So, at this rate, it will take 20 hours to fill the pool with the drain open.