50(115-y) + 25y = 4125       [from eq2]
   5750 - 50y  + 25y = 4125      [distribute]
   5750 - 25y = 4125
   -25y = -1625
     y = 65       [divide both sides by (-25)]
 There are 65 small boxes.
Put that value into either equation (now, which is easier?) to solve for x:
 x = 115 - y
 x = 115 - 65
 x = 50
 There are 50 large boxes.