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 + 160) * 8 + 160 * 6 = 100 * ? + (100 + 200) * 8.
==> 3200 = 100 * ? + 2400.
==> 3200 - 2400 = 100 * ?.
==> ? = 800 / 100.
==> ? = 8.