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.
(100 + ?) * 10 + ? * 6 = 200 * 6 + (200 + 100) * 10.
==> (100 + ?) * 10 + ? * 6 = 4200.
==> Write without parentheses: 100 * 10 + ? * 10 + ? * 6 = 4200.
==> Simplify: ? * 16 + 1000 = 4200.
==> ? * 16 = 4200 - 1000.
==> ? = 3200 / 16.
==> ? = 200