Mechanical advantage is defined as
[tex]MA = \frac{F_{output}}{F_{input}}[/tex]
MA = 4
now we will have
[tex]F_{output} = 200 N[/tex]
as we need to pull up a box of weight 200 N
now by above formula we can find how much force it requires to pull up the box
[tex]4 = \frac{F_{output}}{F_{input}}[/tex]
[tex]4 = \frac{200}{F_{input}}[/tex]
[tex]F_{input} = \frac{200}{4} = 50 N[/tex]
so it requires 50 N force to pull the weight of 200 N
