I need to figure out from where the $ 3 is coming from please

7 Augusta Inc. is a manufacturing company whose total factory overhead costs fluctuate somewhat from month to month. The manager of the company believes that the cost driver is the number of machine-hours used in its production facility. These costs for the last four months in 2020 are given below. Machine-hours Total factory overhead September 6,500 $ 104,400 October 10,000 $ 130,000 November 8,500 $ 118,500 December 5,000 $ 100,000 The factory overhead costs above consist of indirect materials, rent, and maintenance. The company has analyzed these costs at the 5,000 machine-hours level of activity as follows: Indirect materials (variable) $ 10,000 Rent (fixed) ? Maintenance (mixed) 50,000 Total factory overhead costs $ 100,000 Assume that all data are within the relevant range, using the high-low method how much maintenance cost would you expect the company to incur at an operating level of 7200 machine-hours? (Round to the nearest dollars.) A. $ 58,800 B. $ 51,600 C. $ 73,200 D. $ 37,920 E. None of the above Step 1 Fixed cost os fixed at every level of output. Whereas, variable cost varies with the change in output. Step 2 Calculation of expected maintenance cost: Variable cost per unit = Highest cost-Lowest cost Highest activity level-Lowest activity level $130,000-$100,000 10,000-5,000 = $6 Per unit variable cost for indirect material Variable cost of indirect material Machine hour $10,000 5,000 = $2 Per unit variable cost for maintenance = Variable cost per unit = Per units variable cost for indirect material = $6 - $2 = $4 Fixed cost maintenance = $50,000 - (5,000 X 4) = $30,000 Maintenance cost at 7, 200 machine hours = Variable cost + Fixed cost = (7,200 x $3) + $30,000 = $51,600 Therefore, option "b. $51,600" is correct