A steel plate was placed to achieve a steady-state diffusion of gas A. The concentration of this gas A at the high-pressure surface is 6 kg/m3. The diffusion coefficient of this gas A in this steel plate is 4x10-11 m2/s and its diffusion flux in this plate is 1.2x10-7 kg/m2 .s. When the concentration profile of gas A is linear, determine how far (mm) into the plate from the high-pressure side will the concentration be 3 kg/m3