Can you help me figure out how this answer was derived?

The Lumber Division of Paul Bunyon Homes Inc. produces and sells lumber that can be sold to outside customers or within the company to the Construction Division. The following data have been gathered for the coming period. Lumber Division: Capacity 200,000 board feet Price per board foot $2.50Variable production cost per bd. ft. $1.25 Variable selling cost per bd. ft. $0.50 Construction Division: Board feet needed 60,000 Outside price paid per bd. ft. $2.00 If the Lumber Division sells to the Construction Division, $0.35 per board foot can be saved in shipping costs. If the Lumber Division has sufficient excess capacity to fulfill the Construction Division's needs, what will be the effect on the company's overall contribution margin? Decrease by $30,000 Decrease by $24,000 Increase by $36,000 Increase by $33,500