Sheridan Window Cleaners' monthly income statement at several levels of activity is as follows: 2,000 4,600 6,600 $ 3,500 $ 8,050 $ 11,550 1,300 2,990 4,290 2,200 5,060 7,260 Windows washed Sales revenue Cost of goods sold Gross profit Operating expenses Advertising expense Salaries and wages expense Insurance expense Postage expense Total operating expense Operating income 400 400 400 700 960 1,160 200 200 200 1,584 480 1,104 1,780 2,664 420 $ 2,396 $ 3,344 $ 3,916 (a) Identify each expense as fixed, variable, or mixed. Cost of goods sold Advertising expense Salaries and Wages 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 $ - - - - - - 1 1 - 1 $