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 + 160) * 4 + 160 * ? = 120 * 2 + (120 + 180) * 8.
==> 1040 + 160 * ? = 2640.
==> 160 * ? = 2640 - 1040.
==> ? = (1600 / 160.
==> ? = 10.