Question

A 150 kg safe on frictionless casters is being raised at 1.20m to the bed of a truck using planks 4m long. The force needed to push the safe up is

Answer 1

The only thing you need to know in order to solve this task is that plank length (which is force x), should equal the increase in potential energy, so what we have now : (mass)* g * (height).
It has to look like that:  
F * 3.0 = 150 x 9.81 x 1.20 
Then solve for F, the result should be in newtones = 588N

Do hope it makes sense.