Thornton Computer Services, Inc. has been in business for six months. The following are basic operating data for that period: Month Service hours Revenue Operating costs July Aug. 120 136 $6,000 $6,800 $4,300 $5,300 Sept. Oct. Nov. Dec. 260 $13,000 $ 7,100 420 $21,000 $11,200 320 330 $16,000 $16,500 $ 9,100 $10,600 Required a. What is the average service revenue per hour in each month and the overall average for the six-month period? b. Use the high-low method to estimate the total monthly fixed cost and the variable cost per hour. c. Determine the average contribution margin per hour.