Mr. Wu is going to stock the concession stand for the Little League playoffs. He knows he will need at least twice as many hamburger buns as hotdog buns. Hamburger buns cost $0.45 each, and hotdog buns cost $0.40 each. He cannot spend more than $60 on buns. If x = the number of hamburger buns and y = the number of hotdog buns, which system of inequalities could be used to determine how many of each kind of bun Mr. Wu should purchase for the stand?