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 + ?) * 6 + ? * 2 = 100 * 8 + (100 + 140) * 4.
==> (160 + ?) * 6 + ? * 2 = 1760.
==> Write without parentheses: 160 * 6 + ? * 6 + ? * 2 = 1760.
==> Simplify: ? * 8 + 960 = 1760.
==> ? * 8 = 1760 - 960.
==> ? = 800 / 8.
==> ? = 100