A local craftsman named Chuck Synovec builds two kinds of birdhouses, one for wrens and a second for bluebirds. Each wren birdhouse takes 4 hours of labour and 4 units of lumber. Each bluebird house requires 2 hours of labour and 12 units of lumber. The craftsman has available 60 hours of labour and 120 units of lumber. Wren houses yield a profit of $6 each and bluebird houses yield a profit of $15 each.

a) Write out the objective and constraints.

b) Solve graphically.