MAT 540 Linear programming

Globex Investment Capital Corporation owns six companies that have the following estimated
returns (in millions of dollars) if sold in one of the next 3 years:
Company
Year Sold
(estimated returns, $1,000,000s)
1  2  3
1  $14  $18  $23
2  9  11  15
3  18  23  27
4  16  21  25
5  12  16  22
6  21  23  28
To generate operating funds, the company must sell at least $20 million worth of assets in year 1, $25
million in year 2, and $35 million in year 3. Globex wants to develop a plan for selling these companies
during the next 3 years to maximize return.
Formulate an integer programming model for this problem and solve it by using the computer.