The volume of a cylinder is given by the base area times height. (i.e. V = base area x height). Given that the given cylinder is one-third filled with water, the height of the water in the cylinder = 1/2 x height of the cylinder = 1/3 x 12 = 4 m. Therefore, the volume of water in the cylinder = base area of the cylinder x height of water in the cylinder = 100 x 4 = 400 cubic meter.