Aleading manufacturer of household appliances is proposing a self cleaning oven design that involves use of a composite window separating the oven cavity from the room air. the composite is to consist of two high temperature plastics (a and b) of thicknesses xa = 2 xb and thermal conductivities ka = 0.15 w/m. k and kb = 0.08 w/m. k. during the self cleaning process, the oven air temperature is 400°c while the room air temperature is 25°c. the inside and outside convection coefficients are approx. 25 w/m².k. what is the minimum window thickness, x =xa + xb needed to ensure that the outside surface temperature the window remains less than 50°c for safety reasons?