Plate glass, k= 1.35W/m*K, initiallly at 850K, is cooled by blowing air past both surfaces with an effective surface coeffiicent of 5 W/m^2*K. It is necessary, in order that the glass not crack, to limit the maximum temp gradient in the glass to 15 K/cm during the cooling process. At the start of the cooling process, what is the lowest temperature of the cooling air that can be used?