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.
(200 + ?) * 4 + ? * 6 = 100 * 6 + (100 + 100) * 8.
==> (200 + ?) * 4 + ? * 6 = 2200.
==> Write without parentheses: 200 * 4 + ? * 4 + ? * 6 = 2200.
==> Simplify: ? * 10 + 800 = 2200.
==> ? * 10 = 2200 - 800.
==> ? = 1400 / 10.
==> ? = 140