Calculating Mass in Box 3 of a Pulley System

[Henrybar]

Question Summarized:
The box in the middle is remains motionless; how much mass was placed in box 3?

box 1: 35kg
middle box: 95 kg
box 3: ? kg

Attempt at the problem:

 

[mfb]

It would be easier for us if you typed your attempt here.

Check your answer with a calculation for the left mass. Do you see a problem?

The problem statement says the middle mass remains motionless, this does not have to be true for the other masses.

 

[PhanthomJay]

Question Summarized:
The box in the middle is remains motionless; how much mass was placed in box 3?

box 1: 35kg
middle box: 95 kg
box 3: ? kg

you should consider that boxes 1 and 3 might not be at rest.