A square metal block with mass (m) is suspended by five wires with the same length. One of these wires is made of iron at the center of the block and has a radius (1.2 mm) while the other four wires have the same cross-sectional area and they are made of copper at the block corners. If the tension in the iron wire equals (0.3 mg), find the radius of a copper wire. (Take Yiron = 2×10^11, Ycopper = 1. 2×10^11 N/m2 ).