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 + 100) * 4 + 100 * 8 = ? * 6 + (? + 100) * 4.
==> 2000 = ? * 6 + (? + 100) * 4.
==> Write without parentheses: 2000 = ? * 6 + ? * 4 + 100 * 4.
==> Simplify: 2000 = ? * (6 + 4) + 400.
==> 2000 - 400 = ? * 10.
==> (2000 - 400) / 10 = ?.
==> ? = 160