A mining company operates two mines, each of which produces three grades of ores. The West Summit mine can produce two tons of low-grade ore, three tons of medium-grade ore, and one ton of high-grade ore per hour of operation. The North Ridge mine can produce two tons of low-grade ore, one ton of medium-grade ore, and two tons of high-grade ore per hour of operation. To satisfy existing orders, the company needs at least 100 tons of low-grade ore, 60 tons of medium-grade ore, and 80 tons of high-grade ore. If it costs 400 dollars per hour to operate the West Summit mine and 600 dollars per hour to operate the North Ridge mine, how many hours should each mine be operated to supply the needed amounts of ore and, at the same time, minimize the cost of production?
