Solution to Problem 339 | Torsion of thin-walled tube

$T = 2At\tau$
 

Where:
$T = 450 \, \text{lb}\cdot\text{ft} = 450(12) \, \text{lb}\cdot\text{in}$

$A = a^2$

$\tau = 6000 \, \text{psi}$
 

$450(12) = 2a^2(0.10)(6000)$

$a = 2.12 \, \text{ in}$           answer