Heat Conduction question from MHT CET 2025 (19 Apr, Shift 2)

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 $T_2$ and $T_1$ respectively $(T_2 > T_1)$. The rate of heat transfer through the slab in a steady state is $\left[\frac{A(T_2-T_1)K}{x}\right] f$, where $f$ is equal to