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 + 100) * 4 + 100 * 6 = 160 * ? + (160 + 180) * 2.
==> 1640 = 160 * ? + 680.
==> 1640 - 680 = 160 * ?.
==> ? = 960 / 160.
==> ? = 6.