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.
(180 + 180) * 4 + 180 * 8 = 120 * 10 + (120 + ?) * 6.
==> 2880 = 1200 + (120 + ?) * 6.
==> 2880 - 1200 = (120 + ?) * 6.
==> 1680 = (120 + ?) * 6.
==> 1680 / 6 = (120 + ?).
==> (120 + ?) = 280.
==> ? = 280 - 120.
==> ? = 160