Non steady state diffusion.. chemistry...help?

An FCC iron-carbon alloy initially containing 0.55 wt% C is exposed to an oygen-rich and virtually carbon-free atmosphere at 1325 K (1052oC). Under these cirstances the carbon diffuses from the alloy and reacs at the surface with the oxygen in the atmosphere; that is, the carbon concentration at the surface position is maintained at 0 wt% C. (This process of carbon depletion is termed decarburization.) At what position will the carbon concentration be 0.25% after a 10-hour treatment? The value of D at 1325 K is 4.3 x 10-11 m2/s.