Houseco Developers is considering erecting three office buildings. The time (in years) required to complete each of them and the number of workers required to be on the job at all times are shown in the file P06_81.xlsx. After a building is completed, it brings in the following amount of rent per year: building 1,

$50,000; building 2, $30,000; building 3, $40,000.

Houseco faces the following constraints:

■ During each year, 60 workers are available.

■ At most, one building can be started during any year.

■ Building 2 must be completed by the end of year 4.

Determine the maximum total rent that can be earned by Houseco by the end of year 4.