$4\mathrm{.}20$ The same torque is applied on both a solid square shaft of cross section $bb$ and a solid round shaft

of radius $r.$ For both shafts to have equal outer$-$surface maximum shear stress values, what would

be the ratio $\frac{b}{r}?$ For this ratio, compare the weight of the two shafts and also the ratio of strength

to weight$-$see Figure P$4\mathrm{.}20.$

FIGURE P$4.20$