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 + 140) * 8 + 140 * 4 = 200 * 6 + (200 + 120) * ?.
==> 2480 = 1200 + 320 * ?.
==> 2480 - 1200 = 320 * ?.
==> ? = 1280 / 320.
==> ? = 4.