A carpenter quits his job at a furniture factory to open his own cabinetmaking business. In his first two years of operation, his sales average $100 000 per year and his operating costs for wood, workshop and tool rental, utilities, and miscellaneous expenses aver-age $70 000 per year. Now his old job at the furniture factory is again available. What is the lowest wage at which he should decide to return to his old job? Why?