It is found experimentally that the electric field in a certain region of Earth's atmosphere is directed vertically down. At an altitude of 240 m the field has magnitude 60.0 N/C. At an altitude of 180 m, the magnitude is 100 N/C. Find the net amount of charge contained in a cube 60 m on edge, with horizontal faces at altitudes of 180 and 240 m. Neglect the curvature of Earth.