Question

I need help i under stand the adding version just not subtraction

Answer 1

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Define x and y :
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Let x be the number of large boxes.
Let y be the number of smaller boxes.

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Construct equations :
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x + y = 125 -------------------------- (1)
50x  + 30y = 4850 ---------------- (2)

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From equation (1) : 
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x + y = 125
x = 125 - y

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Substitute x = 125 - y into (2) :
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50 (125 - y) + 30y = 4850
6250 - 50y + 30y = 4850
20y = 1400
y = 1400 ÷ 20
y = 70

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Sub y = 70 into equation 1
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x + y = 125
x + 70 = 125
x = 55

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Find the number of boxes :
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Large box = x = 55
Small box = y = 70

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Answer: There are 55 large boxes and 70 small boxes.
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Answer 2

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


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