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 + 120) * 10 + 120 * 6 = 140 * 8 + (140 + ?) * 8.
==> 3520 = 1120 + (140 + ?) * 8.
==> 3520 - 1120 = (140 + ?) * 8.
==> 2400 = (140 + ?) * 8.
==> 2400 / 8 = (140 + ?).
==> (140 + ?) = 300.
==> ? = 300 - 140.
==> ? = 160