In an arc welding operation carried out with a power source maintained at 40 volta and 400 amperes, the consumable electrode melts and just fills the gap between the metal places to be butt-welded. The heat transfer efficiency for the process is 0.8, melting efficiency is 0.3 and he heat required to melt the electrode is 20 J/mm3. If the travel speed of the electrode is 4 mm/s, the cross-sectional area, in mm2, of the weld joint is ___