The temperature of the two outer surfaces of a composite slab, consisting of two materials having coefficients of thermal conductivity

Q: The temperature of the two outer surfaces of a composite slab, consisting of two materials having coefficients of thermal conductivity K and 2K and thickness x and 4x respectively are T₁ and T₂ (T₂ > T₁). The rate of heat transfer through the slab in steady state is $\frac{A(T_2 – T_1)K}{x}$ f , with f  is equal to

(a) 1

(b) 1/2

(c) 2/3

(d) 1/3