Consider the figure shown assuming the green and red blocks have the same weight where should a single green block be placed to make the scale balance

The amount of torque that a mass is based the distance from the fulcrum. The red blocks will create counter-clockwise motion, and the green will create clockwise motion. We need the red and green to be equal.

6 + 4 + 2(2) = 2(3) + 5 + x

14 = 11 + x

x = 3

So the green block should be placed at 3 on top the other two blocks.