$15\mathrm{.}9$ The heat loss from a boiler is to be held at a maximum of $900$Btu $/$ h $*$ f $*$ t $^2$ of wall area. What thickness of asbestos $($k$=$; $0\mathrm{.}1$Btu $/$ h $*$ ft F$)$ is required if the inner and outer surfaces of the insulation are to be $1600$ and $500$ F$,$ respectively? $15\mathrm{.}10$ If$,$ in the previous problem, a $3-$in$-$thick layer of kaolin brick $($k $=0\mathrm{.}07$Btu $/$ h $*$ ftF$)$ is added to the outside of the asbestos, what heat flux will result if the outside surface of the kaolin is $250$ F$?$ What will be the temperature at the interface between the asbestos and kaolin for this condition $?$