Consider a compound slab consisting of two different materials having equal thickness and thermal conductivities K and 2K respectively. The equivalent thermal conductivity of the slabs taken in series is

resistance in series is Rseries=R1+R2 where R = lKA, here l is thickness or length, A is area of cross section,  K is coefficient of thermal conductivity when 2 slabs of different coefficient of thermal conductivity K,2K ,  having same thickness and area of cross section are taken in series effective thermal conductivity is Kseries, given by, l+lKseries=lK+l2K ⇒2Kseries=32K ⇒Kseries=43K