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 + 120) * 10 + 120 * 10 = 200 * 10 + (200 + 100) * ?.
==> 4400 = 2000 + 300 * ?.
==> 4400 - 2000 = 300 * ?.
==> ? = 2400 / 300.
==> ? = 8.