A heated piece of steel, with a uniform initial carbon concentration of 0.20% by mass, was exposed to a carburizing atmosphere for an hour. Throughout the entire process, the carbon concentration on the surface was 0.70%. If the mass diffusivity of carbon in steel in this process was uniform at 1 ×10^–11 m²/s, determine the percentage of mass concentration of carbon at 0.2 mm and 0.4 mm below the surface after the process.