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.
(140 + 120) * 4 + 120 * 8 = 120 * 2 + (120 + ?) * 8.
==> 2000 = 240 + (120 + ?) * 8.
==> 2000 - 240 = (120 + ?) * 8.
==> 1760 = (120 + ?) * 8.
==> 1760 / 8 = (120 + ?).
==> (120 + ?) = 220.
==> ? = 220 - 120.
==> ? = 100