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 + 180) * 4 + 180 * 4 = 160 * 4 + (160 + ?) * 4.
==> 1840 = 640 + (160 + ?) * 4.
==> 1840 - 640 = (160 + ?) * 4.
==> 1200 = (160 + ?) * 4.
==> 1200 / 4 = (160 + ?).
==> (160 + ?) = 300.
==> ? = 300 - 160.
==> ? = 140