please notice it says carbon content at 0.6mm below the surface.

A gear made of 1020 steel (0.20wtC) is to be gas-carburized at 927C(1700F). Calculate the carbon content at 0.6mm below the surface of the gear after a 4.0-h carburizing time. Assume the carbon content at the surface of the gear is 1.00wt%D (C in y iron) at 927C=1.281011m2/s. (Round the final answer to three decimal places.) The carbon content at 0.6mm below the surface of the gear after a 4.0h carburizing time will be wt%