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 + 160) * 4 + 160 * ? = 140 * 6 + (140 + 200) * 6.
==> 1280 + 160 * ? = 2880.
==> 160 * ? = 2880 - 1280.
==> ? = (1600 / 160.
==> ? = 10.