A 20 mm sheet of drywall (thermal conductivity 0.50 W m⁻¹ K⁻¹) is bonded to a 20 mm plank of pine wood (thermal conductivity 0.10 W m⁻¹ K⁻¹). The inner surface of the drywall is at 35 °C, and the outer surface of the wood is at 10 °C. Determine the temperature, in degrees Celsius, at the interface between the two materials under steady-state conditions.