An electric heater is placed inside a room of total wall area $137 {\mathrm{m}}^2$ and maintained at a temperature $20 °\mathrm{C}$ inside, outside temperature $–10 °\mathrm{C}$. The walls are made up of three composite materials. The innermost layer is made up of wood of thickness $2.5 \mathrm{cm}$, the middle layer is of cement of thickness $1 \mathrm{cm}$ and the exterior layer is $25 \mathrm{cm}$ thick. Assuming there is no loss of heat through any other way, the power of the electric heater is (the thermal conductivity of wood, cement and brick are $0.125 \mathrm{W} {\mathrm{m}}^2 °{\mathrm{C}}^{-1}$, $1.5 \mathrm{W} {\mathrm{m}}^{-2} °{\mathrm{C}}^{-1}$ and $1 \mathrm{W} {\mathrm{m}}^{-2} °{\mathrm{C}}^{-1}$ respectively)