$6\mathrm{.}2,6\mathrm{.}3,6\mathrm{.}4$

Each of the sections shown is transmitting a torque of $200lb.in.$ Estimate the maximum shear stress in each section and the angle of twist per unit length.

$($b$)$

$E=30{10}^6$Psi $,v=0\mathrm{.}3$