Two rods identical in geometry but of different materials having co-efficient of thermal expansion α1 and α2 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 increase in temperature. There is no bending of the rods. If α1:α2 = 2: 6 the thermal stresses developed in the rods are equal provided Y1:Y2 is equal to