$T = \dfrac{P}{2\pi f}$
$T_A = T_C = \dfrac{-20(1000)}{2\pi (2)} = -1591.55 \, \text{N}\cdot\text{m}$
$T_B = \dfrac{70(1000)}{2\pi (2)} = 5570.42 \, \text{N}\cdot\text{m}$
$T_D = \dfrac{-30(1000)}{2\pi (2)} = -2387.32 \, \text{N}\cdot\text{m}$
Part (a)
$\tau_{max} = \dfrac{16T}{\pi d^3}$
For AB
$60 = \dfrac{16(1591.55)(1000)}{\pi d^3}$
$d = 51.3 \, \text{mm}$
For BC
$60 = \dfrac{16(3978.87)(1000)}{\pi d^3}$
$d = 69.6 \, \text{mm}$
For CD
$60 = \dfrac{16(2387.32)(1000)}{\pi d^3}$
$d = 58.7 \, \text{mm}$
Use d = 69.6 mm answer
Part (b)
$\theta = \dfrac{TL}{JG}$
$\theta_{D/A} = \dfrac{1}{JG} \Sigma TL$
$\theta_{D/A} = \dfrac{1}{\frac{1}{32}\pi (100^4)(83\,000)} \, [ \, -1591.55(2) + 3978.87(1.5) + 2387.32(1.5) \, ] \, (1000^2)$
$\theta_{D/A} = 0.007\,813 \, \text{rad}$
$\theta_{D/A} = 0.448^\circ$ answer