Question #$3$:

A solid brass bar of diameter $d=1\mathrm{.}2in.$ is subjected to torques $T_1,$ as shown in part $($a$)$ of the figure. The allowable shear stress in the brass is $12$ksi.

$1-$ What is the maximum permissible value of the torques $T_1?$

$2-$ If a hole of diameter $0\mathrm{.}6in.$ is drilled longitudinally through the bar, as shown in part $($b$)$ of the figure, what is the maximum permissible value of the torques $T2?$

$3-$ What is the percent decrease in torque and the percent decrease in weight due to the hole?. Please help me solve the Solid Mechanics question