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) * 10 + 140 * 4 = 140 * 4 + (140 + 100) * ?.
==> 2960 = 560 + 240 * ?.
==> 2960 - 560 = 240 * ?.
==> ? = 2400 / 240.
==> ? = 10.