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.
(120 + 100) * 8 + 100 * 4 = 100 * 4 + (100 + ?) * 8.
==> 2160 = 400 + (100 + ?) * 8.
==> 2160 - 400 = (100 + ?) * 8.
==> 1760 = (100 + ?) * 8.
==> 1760 / 8 = (100 + ?).
==> (100 + ?) = 220.
==> ? = 220 - 100.
==> ? = 120