Two rods of different materials having co- efficient of linear expansion ${\mathrm{\alpha }}_1,{\mathrm{\alpha }}_2$ and Young’s modulii  y₁ and y₂ respectively are fixed between two rigid massive walls. The rods are heated such that they undergo the same increases in temperature. There is no bending of rods. If ${\mathrm{\alpha }}_1:{\mathrm{\alpha }}_2=2:3$ , The thermal stress developed in the two rods are equally provided y₁ / y₂ is equal to.