2 Answers By Expert Tutors ... Let's set x = cost of books and y = cost of pens. ... In algebra, this can be written as x + y = 18. ... Using x and y, we get 1 book and 2 pens costs $21, or x + 2y = 21 ... The pen must cost $3 because we are adding $3 to the total when we ... Please provide a valid phone number.
