The total weight of 15 boxes is 45 pounds .how much would 40 boxes weigh

40 boxes would weigh would weigh 120 pounds

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.