The figure shows the cross section of a wall made of threelayers. The thicknesses of the layers are ????1, ????2 =0.7????1 , ????3 = 0.35????1. The thermal conductivities are????1 , ????2 = 0.9????1 ???????????? ????3 = 0.8????1 . The temperatures at theleft and the right sides of the wall are ???? = 30 °???? and ???? = ?????????15 °???? , respectively. Thermal conduction is steady. What is the temperature difference ????? across the layer 2(between the left and the right sides of the layer)?