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 + 180) * 8 + 180 * 4 = 180 * 4 + (180 + 160) * ?.
==> 3440 = 720 + 340 * ?.
==> 3440 - 720 = 340 * ?.
==> ? = 2720 / 340.
==> ? = 8.