Hot engine oil is being transported in a horizontal pipe (k = 15 W/m∙K, D_i = 5 cm) with a wall thickness of 5 mm. The pipe is covered with a 5-mm-thick layer of insulation (k = 0.15 W/m∙K). A length of 2 m of the outer surface is exposed to cool air at 10°C. If the pipe inner surface temperature is at 90°C, determine the outer surface temperature.