A mining company owns two mines. These mines produce an ore that can be graded into two classes : regular grade and low grade. The company must produce at least 420 tons of regular-grade and 540 tons of low-grade ore per week. The first mine produces 6 tons of regular-grade and 18 tons of low-grade ore per
hour. The second mine produces 18 tons of regular-grade and 9 tons of low-grade ore per hour. The operating cost of the first mine is $26.000 per hour, and the
operating cost of the second mine is $12,500 per hour. The first mine can be operated no more than 40 hours a week, and the second mine can be operated no
more than 38 hours a week. How many hours per week should each mine be operated to minimize the cost?