A Simple "Knapsack" Integer Programming Problem with Binary Variables

StockCo is considering investing up to $21,000 cash in some subset of the following four projects:

  Cash Required Net Return
Investment 1 $7,000 $19,000
Investment 2 $9,000 $21,000
Investment 3 $6,000 $14,000
Investment 4 $5,000 $10,000

Each project is "all or nothing" -- it is not possible to purchase partial shares in a project.  It is also not possible to buy multiple shares.  How can the firm maximize its total net return?