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 + ?) * 2 + ? * 8 = 100 * 8 + (100 + 180) * 4.
==> (160 + ?) * 2 + ? * 8 = 1920.
==> Write without parentheses: 160 * 2 + ? * 2 + ? * 8 = 1920.
==> Simplify: ? * 10 + 320 = 1920.
==> ? * 10 = 1920 - 320.
==> ? = 1600 / 10.
==> ? = 160