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