If only first inlet pipe is running,

In 9 hours, pool can be filled complete.

so, in 1 hour, fraction of pool filled

similarly,

If only second outlet pipe is running,

in 12 hours, pool can be emptied completely.

so, in 1 hour, fraction of pool emptied,

If both pipes are running, fraction of pool filled

so, in 1 hour fraction of pool filled (if both pipes are running)

so, pool can be filled in 36 hours if both pipes running.