3-4.  A coupling with 6 bolts connects the two shafts as shown. The torque  applied on the shafts follows a normal distribution . Each bolt has a diameter, and the bolts are uniformly distributed at a radius . The diameter of the shafts is . Determine the probability that the shear stress in the bolts is less than the maximum shear stress of the shaft. Assume that the shear stress in the bolts is uniform.   (Ans.  )