In a laboratory test of a beam loaded by end couples, the fiber at layer AB as shown are found to increase 30 x 10 mm while those at CD decrease 70 x 10 3 mm in the 280-mm-gage length. Using E = 150 GPa, determine the flexural stress at the bottom fiber. Answer must be in MPa. Given: a = 50 mm, b = 130 mm, and c = 80 mm. %3D %3D aage l ensth