How Do You Solve Newton's Third Law Homework Problems?

[forcefrom]

Alright, so I have this sheet for homework and I'm not sure where to start.

Incase you are unable to see, the questions say:

a) What is the acceleration of the system and the tension in each rope if the girl lifts with a force of 100.0N?
b) What is the force that she pulls with if T3 (bottom bucket) = 15.0N?

http://imageshack.us/photo/my-images/696/photowda.jpg/

To begin, I know I need to utilize the second law's equation F=ma, but I do not know which masses I am supposed to use to find Fg = gm.

I need serious help, I am completely confused!

 

[rock.freak667]

To find the acceleration of the system, you can replace the entire system by an equivalent mass M and a tension T.

So M would be the sum of the 3 masses and T would T1+T2+T3 (they told you what 'T' was in the question.)

Just formulate the resultant force and you should get it now.