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.
(? + 140) * 4 + 140 * 10 = 100 * 6 + (100 + 160) * 8.
==> (? + 140) * 4 + 1400 = 2680.
==> (? + 140) * 4 = 2680 - 1400.
==> (? + 140) = 1280 / 8.
==> ? + 140 = 320.
==> ? = 320 - 140.
==> ? = 180.