hydrogen concentration can take a random value. example solution will suffice

You are hired by ES-Star, one of the most innovative companies in the country. ES-Star is going to produce new generation "low nydrogen" containing steel. The company is going to buy the steel from Kardemir either in square billet or cylindrical bar form. The billets are 80mmx80mmx4m (i.e., length) and the bars are 50mm in diameter and 4m long. The initial hydrogen concentration fort he billet and the bar are 21017 hydrogen atoms per cm3 and they are going to be subjected to the same vacuum outgassing treatment (where constant surface hydrogen concentration is 21015 atoms. cm3 ) at a temperature at which Dr:2.0109m2s1. If the minimum outgassing treatment time for the billet is 30 hours then calculate the maximum allowable hydrogen content in ES-Star's products. (Hint: You need to use the related figure, given in the Appendix in order to solve this problem) Calculate the minimum outgassing treatment time to be required to reduce the hydrogen concentration to the above mentioned level, fort he cylindrical bars? If the costs of billets and bars to ES-Star are $50/ piece and $150/ piece, respectively and the cost for vacuum outgassing treatment is $5/h, then which type of steel geometry (i.e., billet or bar) should be bought from Kardemir and treated by ES-Star in order to make ES-Star's process much more profitable? You are the New Engineer who should help the company to gain more money. (Assume that; at each time, only one piece of the billet or the bar is treated in the vacuum outgassing chamber, Sale price for both geometries (in per piece) are the same)