To answer this, we must first assume that each box weighs the same. If this is true, you would make a rate of how many pounds in 15 boxes: 45 lbs/15 boxes. From here determine the weight per box by dividing 45 by 15. This means each box is 3 pounds. Multiply this by the total number of boxes (3 x 40). This gives you a total weight of 120 pounds for 40 boxes.