An electric hot plate is maintained at a temperature of $350C$ and is used to keep a solution boiling at $95C.$ The solution is contained in a cast iron $(k=50\frac{W}{m}K)$ vessel of wall thickness $25mm$ which is enamelled to a thickness of $0\mathrm{.}8mm.$ The heat transfer coeffi cient for the boiling solution is $5\mathrm{.}5k\frac{W}{?}m2K$ and thermal conductivity of enamel is $1\mathrm{.}05\frac{W}{m}K.$ Calculate the overall heat transfer coefficient $.$