Resolving forces

[Vikash]

In the 6th question, How do we know whether the trailer is being pulled or pushed by the force of 20N?
I tried both ways but get different answers for the coefficient of friction?? How do I approach this?......calculation is easy, I just want to know HOW DO WE KNOW in which direction the force of 20N acts (pulls or pushes?) the trailer.

 

[Harry_the_cat]

It shouldn't affect the answer. Show your working for both methods so we can see what's gone wrong.