An fcc iron-carbon alloy initially containing 0.55 wt% c is exposed to an oxygen-rich and virtually carbon-free atmosphere at 1052 k (1127°c). under these circumstances the carbon diffuses from the alloy and the reacts at the surface with the oxygen in the atmosphere, that is, the carbon concentration at the surface position is maintained essentially at 0 wt% c. (this process of carbon depletion is termed as decarburization) at a distance of 1.26 mm from the surface, the carbon concentration is 0.15 wt% after a 10-h treatment.

at what position will the carbon concentration be 0.15 wt% after a 20-h treatment?