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 * ? = 140 * 8 + (140 + 160) * 8.
==> 2800 + 120 * ? = 3520.
==> 120 * ? = 3520 - 2800.
==> ? = (720 / 120.
==> ? = 6.