A homeowner has installed two systems of gutters and 100-liter barrels to collect water that runs off of her roof, one system on the north side and the other on the south side. Sixty percent of the water that falls on her roof runs into the gutters of the north side system and the remaining 40 percent runs into the south side gutters. All water then drains into the barrels. No barrel is attached to both of the gutter systems, and all barrels contain only water that has drained from the gutters.