Sheridan Window Cleaners' monthly income statement at several levels of activity is as follows: Windows washed 2,000 4,600 6,600 Sales revenue $ 3,500 $ 8,050 $ 11,550 Cost of goods sold 1,300 2,990 4,290 Gross profit 2,200 5,060 7,260 Operating expenses Advertising expense 400 400 400 Salaries and wages expense 700 960 1,160 Insurance expense 200 200 200 Postage expense 480 1,104 1,584 Total operating expense 1,780 2,664 3,344 Operating income $ 420 $ 2,396 $ 3,916 %24 (a) Identify each expense as fixed, variable, or mixed. Cost of goods sold Advertising expense Salaries and VWages expense Insurance expense Postage expense (b) Prepare a contribution margin income statement based on a volume of 5,100 windows. (Round unit cost to 2 decimal places, e.g. 52.75 and all other answers to O decimal places, e.g. 5,275.) 5,100 windows Per Unit %24 %24 %24