3-3.  A coupling connects the two shafts as shown. The torque  applied on the shafts follows a normal distribution of . Each bolt has a diameter , and the bolts are uniformly distributed at the radius . If the allowable shear stress of the bolt is , determine how many bolts the coupling should have to make sure that the probability of failure of each bolt is less than . Assume that  and  are independent, and the shear stress in the bolts is uniform.     (Ans.  )