Each pair of boxes weighs 40 pounds + 10 pounds, or <<40+10=50>>50 pounds.
Each pair weighs 50 pounds, so the delivery trucks can carry 2000 / 50 pairs of boxes or <<2000/50=40>>40 pairs.
Steven has three trucks, so each trip can move 40 * 3 pairs of boxes = <<40*3=120>>120 pairs of boxes.
Each pair is a set of two boxes, so each delivery contains 120 * 2 boxes = <<120*2=240>>240 boxes
#### 240