Block pushed up against wall, determine the force of friction

[isukatphysics69]

1. Homework Statement
I have already solved this one weeks ago, and i have a test Tuesday so i was reviewing these problems.
I understand everything when solving with 60N as the force.. but then when i changed the force to 20N,
i had to use the coefficient of kinetic friction to get the correct answer.. how exactly would i know if the block is moving just by changing the force from 60N to 20N? There should be a certain threshold where between x and y Newtons of force the block stays stationary and should only use static friction coefficient..I completely forget how i should figure on an exam whether i should use kinetic or not given this situation.. i am asking conceptually here how should i know?

Homework Equations

f=ma

The Attempt at a Solution

asking conceptually here. have already solved

 

[haruspex]

There should be a certain threshold where between x and y Newtons of force the block stays stationary and should only use static friction coefficient

That is what the static coefficient is for, it tells you the threshold, and nothing more.
Under static friction, |F_friction|≤μ_sN.
Under kinetic, |F_friction|=μ_kN.
If the net of the other forces parallel to the surface is under the threshold then the force is equal and opposite to that; if it exceeds the threshold then you have kinetic friction.
In each case, the direction must be determined by asking what the relative motion of the surfaces would be in the absence of friction.