An iron$-$carbon alloy initially containing $0\mathrm{.}315wt\%C$ is exposed to an oxygen$-$rich and virtually carbon$-$free atmosphere at $1120C.$ 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 $0\mathrm{.}0wt\%C.$ At what position will the carbon concentration be $0\mathrm{.}236$ wt $\%$ after a $9h$ treatment? The value of $D$ at $1120C$ is $2\mathrm{.}8{10}^{-10}\frac{m^2}{s}.$

$\backslash$table$[[z,$erf$(z),z,$erf$(z),z,$erf$(z)$