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.
(160 + 200) * ? + 200 * 6 = 160 * 8 + (160 + 100) * 8.
==> 360 * ? + 1200 = 3360.
==> 360 * ? = 3360 - 1200.
==> ? = 2160 / 360.
==> ? = 6.