At a library book sale, there is one price for hardback books
and different price for paperback books.
Last week, 40 hardbacks (x) and 60 paperbacks (y) were
sold, for a total of $380.
This week, 60 hardbacks (x) and 120 paperbacks (y) were
sold, for a total of $600.
Use the equation you identified in question 4 for this system
and solve to determine the cost of each hardback book and
each paperpack book.