A moving van can safely carry up to 4,800 pounds. It must transport a 150-pound dolly and a mix of boxes; some boxes weigh 20 pounds each and some weigh 60 pounds each. Let $a$ be the number of 20-pound boxes and $b$ be the number of 60-pound boxes. Which inequality represents the possible combinations of boxes the van can carry during one trip with only the dolly and the boxes?