Figure shows ( in cross section ) a wall consisting of four layers, with thermal conductivities `k_(1) = 0.060W//m.K, k_(3)= 0.040W//m.K`, and `k_(4) = 0.12W//m.K(k_(2)` is not known). The layer thickness are `L_(1) = 1.2 cm, L_(3) = 5.6 cm`, and `L_(4) = 4.0 cm` `( L_(2)` is not known ) . The known temperatures are `T_(1) = 30^(@)C, T_(12) = 25^(@)C`, and `T_(4) = - 10^(@) C` . Energy transfer through the wall is steady. What is interface temperature `T_(34)` ?

Answer

`-1.7^(@)C`