Well he and the cart together weigh: 210+95=305.
The weight left for boxes is: 4,000-305=3,695.
Total weight of boxes is: 50·275=13,750.
Divide the weight of boxes by how much is left for the elevator: 13,750/3,695=3.72
Since 3.72 is more than 3 he would have to make 4 trips to safely do so :)
