A brick wall $($thermal conductivity $k=0\mathrm{.}8\frac{W}{m}*K$ is $0\mathrm{.}3$ m thick and separates a furnace $($inner surface$)$ maintained at $800C$ from the surrounding environment at $25C.$ The area of the wall is $15m^2.$ The heat transfer coefficient on the outer surface due to natural convection is $h=10\frac{W}{m^2}*K.$ Assume steady$-$state conditions.

a$)$ Calculate the total thermal resistance of the system, considering conduction through the wall and convection on the outer surface.

b$)$ Determine the heat transfer rate through the wall.

c$)$ Find the temperature at the outer surface of the wall.