Two slabs are of the thicknesses $ d_1 $ and $ d_2 $. Their thermal conductivities are $ K_1 $ and $ K_2 $, respectively. They are in series. The free ends of the combination of these two slabs are kept at temperatures $ \theta_1 $ and $ \theta_2 $. Assume $ \theta_1>\theta_2 $. The temperature $ \theta $ of their common junction is