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.
(140 + 200) * 6 + 200 * 10 = ? * 4 + (? + 180) * 10.
==> 4040 = ? * 4 + (? + 180) * 10.
==> Write without parentheses: 4040 = ? * 4 + ? * 10 + 180 * 10.
==> Simplify: 4040 = ? * (4 + 10) + 1800.
==> 4040 - 1800 = ? * 14.
==> (4040 - 1800) / 14 = ?.
==> ? = 160