Jake is comparing the prices of two mattress cleaning companies. Company A charges $30 per mattress and an additional $13 as service charges. Company B charges $28 per mattress and an additional $15 as service charges. Part A: Write an equation to represent each company's total charges for cleaning a certain number of mattresses. For both equations (one for Company A and one for Company B), define the variable used. (4 points)

Part B: Which company would charge less for cleaning 4 mattresses? Justify your answer. (3 points)
Part C: How much money is saved by using the services of Company B instead of Company A to clean 7 mattresses?