A craftsman builds two types of birdhouses, one for wrens and a second for bluebirds. Each wren birdhouse takes 4 hours of labor and 4 units of lumber. Each bluebird birdhouse takes 2 hours of labor and 12 units of lumber. The craftsman has available 60 hours of labor and 120 units of lumber. Wren houses yield a profit of $6 each and bluebird houses yield a profit of $ 15 each. Formulate (objective function and constraints) the problem as an LP problem.