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 + 120) * 2 + 120 * 4 = ? * 2 + (? + 180) * 2.
==> 920 = ? * 2 + (? + 180) * 2.
==> Write without parentheses: 920 = ? * 2 + ? * 2 + 180 * 2.
==> Simplify: 920 = ? * (2 + 2) + 360.
==> 920 - 360 = ? * 4.
==> (920 - 360) / 4 = ?.
==> ? = 140