Concord Window Cleaners' monthly income statement at several levels of activity is Windows washed 2,000 4,2008,200 Sales revenue $ 3,900 $ 8,190 $ 15,990 Cost of goods sold 1,400 2,940 5,740 Gross profit 2,500 5,250 10,250 Operating expenses Advertising expense 400 400 400 Salaries and wages expense 700 920 1,320 Insurance expense 200 200 200 Postage expense 450 945 1,845 Total operating expense 1,750 2,465 3,765 Operating income $ 750 $ 2,785 $ 6,485 (a) Identify each expense as fixed, variable, or mixed. variable Cost of goods sold Advertising Salaries and Wages fixed mixed A Insurance fixed Postage variable 6,400 windows Per Unit