What effort will be required to move a 500 n load using a 4 m long lever with a fulcrum 1.5 m from the load?
1875 n
133 n
300 n
833 n

Use the fulcrum is your reference point. In this question, they didn't specify which side the load (F) is so you could pick the left or right side.  

assuming the load is on the left side of the fulcrum,  

the distance of the load from the fulcrum= d₁

F₁=500 N

d₁=1.5 m (given in the question)

The right side of the fulcrum:

d₂= 4m - 1.5m = 2.5 m (distance from fulcrum to other load)

F₂= what you're trying to find

W=fd

Work on left side = F₁ x d₁ = 500N x 1.5 m=750 Nm

Work on right side = F₂ x d₂ = F₂ x 2.5 m

Work on left hand side= work on right hand side

750 Nm = F₂ x 2.5 m

F₂= 750 Nm/ 2.5 m= 300 N

The process is the same if you assume the load is on the right just swap everything done on the left here as the right, and the right as left.

the minimum amperage that the current in this wire would have to have so that its weight is completely supported by the magnetic force due to earth's field, assuming that no other forces except gravity act on it