Answer:
The box should be placed at a distance of
from the pivot
Step-by-step explanation:
In order to be in static equilibrium, both Torques have to be the same magnitude, so:
Replacing the formula for Torque:
where X is the distance we need to find.
Solving for X we get:

As we can see, the distance does not depend on the actual value of the mass but on the fact of one being twice as much as the other one.