An FCC iron$$carbon alloy initially containing $0\mathrm{.}4$ wt$\%$ C is exposed to an oxygen$-$rich and virtually carbon$-$free atmosphere at $1300$ K $.$ Under these circumstances the carbon diffuses from the alloy and reacts at the surface with the oxygen in the atmosphere$$that is$,$ the carbon concentration at the surface position is maintained essentially at $1$ wt$\%$ C$.($This process of carbon depletion is termed decarburization.$)$ At what position will the carbon concentration be $0\mathrm{.}25$ wt$\%$ after a $10-$h treatment? The value of D at $1325$ K is $3\mathrm{.}3\backslash$times $1011$ m$2/$s$.$