The thermal conductivity of brick is `1.7 W m^(-1) K^(-1)`, and that of cement is `2.9 W m^(-1) K^(-1)`.What thickness of cement will have same insulation as the brick of thickness `20 cm`.

Explanation

Since `Q= (KA(T_(1)-T_(2))t)/(L)`. For same insulation by the brick and cement `Q, A(T_(1)-T_(2))` and t do not change.
Hence , `(K)/(L)` remain constant . If `K_(1)` and `K_(2)` be the thermal conductivity of brick and cement respectively `L_(1)` and `L_(2)` be the required thickness then `(K_(1))/(L_(1)) = (K_(2))/(L_(2))` or `(1.7)/(20) = (2.9)/(L_(2))` `therefore` `" "` `L_(2) = (2.9)/(1.7) xx 20 = 34.12 cm`