The temperature of the outer surface of a composite slab, consisting of two materials having coefficients of thermal conductivity \(K\) and \(2K\) and thickness \(x\) and \(4x\), respectively are \(T_2\) and \(T_1(T_2>T_1)\). The rate of heat transfer through the slab, in a steady state is \(\left(\dfrac{A(T_2-T_1)K}{x}\right)f\), with \(f\) equal to