Due to equilibrium of moments:
1) The weight of the person hanging on the left is 250 N
2) The 400 N person is 3 m from the fulcrum
3) The weight of the board is 200 N
Explanation:
1)
To solve the problem, we use the principle of equilibrium of moments. 
In fact, for the seesaw to be in equilibrium, the total clockwise moment must be equal to the total anticlockwise moment.
The moment of a force is defined as:

where
F is the magnitude of the force
d is the perpendicular distance of the force from the fulcrum
In the first diagram:
- The clockwise moment is due to the person on the right is

where  is the weight of the person and
 is the weight of the person and  is its distance from the fulcrum
 is its distance from the fulcrum
- The anticlockwise moment due to the person hanging on the left is

where  is his weight and
 is his weight and  is the distance from the fulcrum
 is the distance from the fulcrum
Since the seesaw is in equilibrium,

So we can find the weight of the person on the left:

2)
Again, for the seesaw to be in equilibrium, the total clockwise moment must be equal to the total anticlockwise moment.
- The clockwise moment due to the person on the right is

where  is the weight of the person and
 is the weight of the person and  is its distance from the fulcrum
 is its distance from the fulcrum
- The anticlockwise moment due to the person on the left is

where  is his weight and
 is his weight and  is the distance from the fulcrum.
 is the distance from the fulcrum.
Since the seesaw is in equilibrium,

So we can find the distance of the person on the right:

3)
As before, for the seesaw to be in equilibrium, the total clockwise moment must be equal to the total anticlockwise moment.
- The clockwise moment around the fulcrum this time is due to the weight of the seesaw:

where  is the weight of the seesaw and
 is the weight of the seesaw and  is the distance of its centre of mass from the fulcrum
 is the distance of its centre of mass from the fulcrum
- The anticlockwise moment due to the person on the left is

where  is his weight and
 is his weight and  is the distance from the fulcrum
 is the distance from the fulcrum
Since the seesaw is in equilibrium,

So we can find the weight of the seesaw:

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