An iron-carbon alloy initially containing 0.345 wt% c is exposed to an oxygen-rich and virtually carbon-free atmosphere at 1050°c. 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 0.0 wt% c. at what position will the carbon concentration be 0.259 wt% after a 6 h treatment? the value of d at 1050°c is 5.6 × 10-10 m2/s.