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.
(140 + 200) * 8 + 200 * 2 = 160 * 2 + (160 + 120) * ?.
==> 3120 = 320 + 280 * ?.
==> 3120 - 320 = 280 * ?.
==> ? = 2800 / 280.
==> ? = 10.