Two rods of different materials having coefficients of thermal expansion α₁, α₂ and young’s modulus Y₁, Y₂ respectively are fixed between two rigid walls. The rods are heated such that they undergo the same increase in temperature. There is no bending of rods. If α₁ : α₂ =2:3, thermal stress developed in the rods are equal provided Y₁ : Y₂ is equal to