The shaft is made of $\mathrm{A} 992$ steel with the allowable shear stress of $\tau_{\text {allow }}=75$ MPa. If gear $B$ supplies $15 \mathrm{kW}$ of power, while gears $A, C$ and $D$ withdraw $6 \mathrm{kW}, 4 \mathrm{kW}$ and $5 \mathrm{kW}$, respectively, determine the required minimum diameter $d$ of the shaft to the nearest millimeter. Also, find the corresponding angle of twist of gear $A$ relative to gear $D$. The shaft is rotating at 600 rpm.