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Answer:

Explanation:
Since the two charged bodies are symmetric, we can calculate the electric field taking both of them as point charges.
This can be easily seen if we use Gauss's law, 
We take a larger sphere of radius, say r, as the Gaussian surface. Then the electric field due to the charged sphere at a distance r from it's center is given by,

which is the same as that of a point charge.
In our problem the charges being of opposite signs, the electric field will add up. Therefore,

where,
= distance between the center of one sphere to the midpoint (between the 2 spheres)
The half-meter rule (easy math) is 0.5 meters or 50 centimeters since a meter is 1 meters long, which is equivalent to 100 centimeters. Therefore, we shall apply the 50 cm rule.
A 50 cm rule's center of mass is now 25 cm away.
Additionally, according to the data, the object is pivoted at 15 cm, while the 40 g object is hung at 2 cm from the rule's beginning. Using a straightforward formula, we can compare the two situations: the distance from the pivot to the center of the mass times the mass of the 40 g object divided by 2 cm must equal the distance from the pivot to the center of the mass times mass of the 10 x g object
The result of the straightforward computation must be 52g.
Most simplified version:
the center of mass of the rule is at the 25 cm mark
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