Determine the minimum height h of an 80 mm wide beam if the flexural stress is not to exceed 20 MPa: the beam is 4 m long with a UDL of 2.5 kN/m over its whole length, supports at 1 m and 4 m from the left end, and a 5 kN load at 2 m from the right end. Give M_{max} (kN·m) and h (mm).