Ineed i under stand the adding version just not subtraction a delivery truck is transporting boxes of two sizes: large and small. the large boxes weigh 50 pounds each, and the small boxes weigh 30 pounds each. there are 125 boxes in all. if the truck is carrying a total of 4850 pounds in boxes, how many of each type of box is it carrying?

Let L and S represent the numbers of Large and Small boxes, respectively.
.. L + S = 125 . . . . . . . . . . there are 125 boxes in all
.. 50L + 30S = 4850 . . . . . total weight of all boxes is 4850 pounds

You can multiply the first equation by -30 and add that to the second.
.. -30L -30S = -3750
Adding this to the second equation, we get
.. (-30L +50L) +(-30S +30S) = (-3750 +4850)
.. 20L = 1100
.. L = 55

and from the first equation ...
.. 55 + S = 125
.. S = 70

The truck is carrying
.. 55 large boxes
.. 70 small boxes


Subtraction and addition are basically the same thing. You just need to pay attention to the sign of the number.