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) * 8 + 160 * 8 = 180 * 10 + (180 + ?) * 6.
==> 3840 = 1800 + (180 + ?) * 6.
==> 3840 - 1800 = (180 + ?) * 6.
==> 2040 = (180 + ?) * 6.
==> 2040 / 6 = (180 + ?).
==> (180 + ?) = 340.
==> ? = 340 - 180.
==> ? = 160