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 + ?) * 10 + ? * 6 = 200 * 2 + (200 + 200) * 8.
==> (200 + ?) * 10 + ? * 6 = 3600.
==> Write without parentheses: 200 * 10 + ? * 10 + ? * 6 = 3600.
==> Simplify: ? * 16 + 2000 = 3600.
==> ? * 16 = 3600 - 2000.
==> ? = 1600 / 16.
==> ? = 100