Coal is mined and processed at the following four mines in Kentucky, West Virginia, and Virginia:
Location .... Capacity (tons)
A. Cabin Creek ... 90
B. Surry ..... 50
C. Old Fort ..... 80
D. McCoy .... 60
280
These mines supply the following amount of coal to utility power plants in three cities:
Plant ...... Demand (tons)
1. Richmond ... 120
2. Winston-Salem . 100
3. Durham .... 110
330

The railroad shipping costs (in thousands of dollars) per ton of coal are shown in the following table. Because of railroad construction, shipments are prohibited from Cabin Creek to Richmond:

Formulate this problem as a linear programming model and solve it by using thecomputer.

To (cost, in $1,000s) From $ 7 12 $ 5 4 $10