Open the water outlet pipe 12 minutes to drain all the water in the pool, showing that the water outlet speed =112.
When the water inlet and outlet are opened at the same time, the water injection speed =1/8-112 =1/24.
If the water inlet pipe is opened first 1 min, the pool has been filled with water = 1/8, and there is no water in 7/8 pools. rule
(1-1/8)/(1/8-112) = 21minute.
It takes 2 1 minute to fill the pool.