Two rods of different materials having coefficients of thermal expansion α1and α2 and Young's moduli Y1 and 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 a1 and a2 are in the ratio 2 : 3, the thermal stresses developed in the two rods are equal provided Y1 : Y2 is equal to: