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 + 160) * 4 + 160 * 2 = 200 * 4 + (200 + ?) * 2.
==> 1520 = 800 + (200 + ?) * 2.
==> 1520 - 800 = (200 + ?) * 2.
==> 720 = (200 + ?) * 2.
==> 720 / 2 = (200 + ?).
==> (200 + ?) = 360.
==> ? = 360 - 200.
==> ? = 160