A carpenter makes tables and chairs. Each table can be sold for a profit of $50 and each chair for a profit of $20. The carpenter can afford to spend up to 42 hours per week working and takes 5 hours to make a table and 4 hours to make a chair. Customer demand requires that he makes at least three times as many chairs as tables. Tables take up 3 times as much storage space as chairs and there is room for at most four tables each week. Formulate this problem as a linear programming problem.