Question 1201056
The combined weight of a large box and a small box is 90 pounds.

Let the small box weigh x pounds
and large box weigh weigh y pounds
Equating the numbers
x+y =90 ....................1

 The truck is transporting 65  large boxes and 70 small boxes. The truck is carrying a total of 6000 pounds in  boxes,
Equating the weight 
70x +65y =6000 ..............2

x+y =90  ( Multiply by -70
-70x-70y = -6300
70x +65y =6000
Add both equations
-5y = -300
y= 60 pounds
Balancex= 30 pounds