in january, the left coast bookstore chain sold 700 hardcover books, 1,800 softcover books, and 2,000 plastic books in san francisco; it sold 300 hardcover, 300 softcover, and 500 plastic books in los angeles. hardcover books sell for $30 each, softcover books sell for $10 each, and plastic books sell for $15 each. suppose that each hardcover book costs the stores $10, each softcover book costs $5, and each plastic book costs $10. use matrix operations to compute the total profit at each store in january.