Then open the inlet pipe and outlet pipe at the same time, and put in 1/60 pool water every minute.

The water inlet pipe injects 1/90 pool water every minute.

Then the outlet pipe will discharge1/60+1/90 =1/36 every minute (outlet pipe-inlet pipe = inlet pipe and outlet pipe are opened at the same time).

Open the water outlet pipe separately1(1/36) = 36 (minutes) to drain the water in the pool.

If you don't understand, please ask. If it is helpful, please adopt it. Thank you!