The scale is balanced. This is the case if the distances * weights are the same on both sides of the scale: (A + B) * weigth 1 + B * weight 2 = C * weigth 3 + (C + D) * weight 4.
(100 + 140) * 8 + 140 * 4 = ? * 6 + (? + 120) * 4.
==> 2480 = ? * 6 + (? + 120) * 4.
==> Write without parentheses: 2480 = ? * 6 + ? * 4 + 120 * 4.
==> Simplify: 2480 = ? * (6 + 4) + 480.
==> 2480 - 480 = ? * 10.
==> (2480 - 480) / 10 = ?.
==> ? = 200