The wall of drying oven is constructed by sandwiching insulation material of thermal conductivity k = 0.05 w/m°k between thin metal sheets. the oven air is at = 300℃, and the corresponding convection coefficient is ℎ = 30 w/ m2 °k. the inner wall absorbs a radiant flux of ′′= 100 w/ m2 from hotter objects within the oven. the room air at t[infinity],o= 25℃, and overall coefficient for convection and radiation from the outer surface is ℎo = 10 w/ m2 °k. what insulation thickness l is required to maintain the outer wall surface at a safe-to-touch temperature of to = 40 degree c?