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 + 120) * 8 + 120 * 8 = 160 * 10 + (160 + 120) * ?.
==> 2720 = 1600 + 280 * ?.
==> 2720 - 1600 = 280 * ?.
==> ? = 1120 / 280.
==> ? = 4.