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 * ? = 180 * 8 + (180 + 100) * 2.
==> 1040 + 160 * ? = 2000.
==> 160 * ? = 2000 - 1040.
==> ? = (960 / 160.
==> ? = 6.