Milkem Asset Sales Problem:
 Binary Variable Grids

Boris Milkem owns six assets that he must sell over the next three years.  He expects that the sales will generate the following revenue, in millions of $

  Year 1 Year 2 Year 3
Asset 1 15 20 24
Asset 2   16 18 21
Asset 3 22 30 26
Asset 4 10 20 30
Asset 5 17 19 22
Asset 6 19 25 29

For example, if he sells asset 3 in year 2, he receives $30 million.  Each asset can be sold at most once, of course.  He must sell at least $20 million of assets in year 1, $30 million in year 2, and $35 million in year 3.

How can he maximize his total revenue from the sales?