The assembly shown is composed of three different diameter rods of three different materials. When the temperature is $\backslash (70^\{\backslash$circ$\}\backslash$mathrm$\{$F$\}\backslash ),$ the assembly fits perfectly between its fixed supports at either end. Determine the average normal stress in each material when the temperature reaches $\backslash (120^\{\backslash$circ$\}\backslash$mathrm$\{$F$\}\backslash ).$