$2\mathrm{.}3.$ A furnace wall consists of three insulating materials in series. $32$ per cent

chrome brick, magnesite bricks, and low$-$grade refractory bricks $(k=0\mathrm{.}5).$ The

magnesite bricks cannot withstand a face temperature above $1500F,$ and the low$-$

grade bricks cannot exceed $600F.$ What thickness of the wall will give a heat loss

not in excess of $1500Bt\frac{u}{hr}(f{t}^2)$ when the extreme face temperatures are $2500$ and

$200F,$ respectively?