One surface of an iron plate at $700$ C $(973$ K$)$ is subjected to carbon$-$rich carburization.

The other surface was exposed to the carbon$-$poor decarburization atmosphere.

In case of steady diffusion, $5$ and $10$ mm $(7$ x $10-3$ and $10-2$ m$)$ from the carburizing surface

Since the carbon concentrations in the air are $1\mathrm{.}4$ and $0\mathrm{.}6$ kg$/$m$3,$ respectively,

How many kg$/$m$2$s is the diffusion flux of carbon in the iron?