A straight railway AB is 200km long, running from east to west. A
factory C is situated 30km due north of the point A. Now a straight
highway is to be built from a point D on AB to C. Assume that the
cost for transporting goods per ton per kilometer by railway is
4/5 as much as the cost by highway. In order to minimize the cost of transporting raw materials from the supply station B to the factory C,
where should D be located on AB?