An empty swimming pool can be filled to capacity through an innlet pipe in 3 hours and it can be completely drained by a drained pipe in 6 hours if both pipe are fully open at the same time in how man?

Assuming the flow rate is constant (in a real system, this probably will not be the case, at least for the drain; it probably drains faster when nearly full than when nearly empty), then for convenience sake let's say the pool holds six units of water. It fills at 2 units per hour and drains at 1 unit per hour. If both pipes are open, the net gain is 1 unit per hour, so it will fill in six hours (and then start overflowing unless both pipes are shut off).