A 2.0-mm-thick sheet of polyethylene covers a 10-cm-thick slab of high-density particle board that is perfectly insulated on the back side. The assembly is initially uniform in temperature at 20ºC. If the outer surface of the plastic is suddenly exposed to a constant heat flux of 1300W/m2, estimate how long will it take for the insulated back surface to reach a temperature of 50ºC. State your assumptions.