Hiwatha Manufacturing has two factories that produce three grades of paper: low grade, medium grade, and high grade. It needs to supply at least 42 tons of low grade, 30 tons of medium grade, and
12 tons of high grade paper. Factory A produces 3 tons of low grade, 1 ton of medium grade, and 3
tons of high grade paper daily, and costs $1200 per day to operate. Factory B produces 6 tons of low
grade, 6 ton of medium grade, 1 tons of high grade paper daily, and costs $1800 per day to operate.
How many days should each factory operate to fill the orders at minimum cost?