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 + 200) * 6 + 200 * 8 = 120 * 8 + (120 + ?) * 10.
==> 3760 = 960 + (120 + ?) * 10.
==> 3760 - 960 = (120 + ?) * 10.
==> 2800 = (120 + ?) * 10.
==> 2800 / 10 = (120 + ?).
==> (120 + ?) = 280.
==> ? = 280 - 120.
==> ? = 160