A composite slab consists of two materials having coefficients of thermal conductivity $K$ and $2K$, thickness $x$ and $4x$ respectively. The temperatures of two outer surfaces of a composite slab are $\text{T}_2$ and $\text{T}_1$ respectively ($\text{T}_2 > \text{T}_1$). The rate of heat transfer through the slab in a steady state is $\left[ \frac{A(\text{T}_2-\text{T}_1)K}{x} \right] f$, where $f$ is equal to}
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Thermal conductivity is analogous to electrical resistance: $R = L/(KA)$. Series sum is $R_1 + R_2$.