GATE Mechanical Engineering (ME) 2018 Shift 2 Solved Paper

A hollow circular shaft of inner radius $10\mathit{m}\mathit{m}$, outer radius $20\mathit{m}\mathit{m}$ and length $1\mathit{m}$ is to be used as a torsional spring. If the shear modulus of the material of the shaft is 150 GPa, the torsional stiffness of the shaft (in $\mathit{k}\mathit{N}-\mathit{m}/\mathit{r}\mathit{a}\mathit{d}$ ) is___________ (correct to two decimal places).