a solid cylindral shaft is subjected to torque of $28647\mathrm{.}8$Nm

$1.$if the shear stress is not to exceed $80$N$/$mm$?$ find its diameter

$2.$ What percent saving in weight would be obtained if this shaft is replaced by a hollow one whose internal diameter equal to $0\mathrm{.}6$ of the external diameter, the length, the material and the maximum shear stress being the same

show your calculation on paper please as soon as possible