The total cost of four pens and seven mechanical pencils is $13.25. the cost of each pencil is 75 cents. a. using an arithmetic approach, find the cost of a pen. b. let the cost of a pen be dollars. write an expression for the total cost of four pens and seven mechanical pencils in terms of . c. write an equation that could be used to find the cost of a pen. d. determine a value for for which the equation you wrote in part (b) is true.