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 T2 and T1 (T2 > T1 ). The rate of heat transfer through the slab, in a steady state is (A(T2 - T1)K/x)f , with f equal to
(a) 1
(b) 1/2
(c) 2/3
(d) 1/3.
