the torque T must be reduced by

Question

A solid shaft of diameter 100 mm, length 1000 mm is subjected to a twisting moment T. The maximum shear stress developed in the shaft is 60 N/mm² . A hole of 50 mm diameter is now drilled throughout the length of the shaft. To develop a maximum shear stress of 60 N/mm² in the hollow shaft, the torque T must be reduced by