What is the entropy if you have a spray can with 30psi?

I am asking this question because I am trying to understand what entropy is and I just can't seem to get it clear.

Now I think the asnwer is 0.5.
The pressure of the can is 30.
the Atmospheric pressure is around 15.
You devide the pressure in the can by the atmospheric pressure and you have the entropy.

So first question is, is that right?
Second question is what is the system?
Is the system the can or the can and the atmospheric pressure or just the atmosheric pressure?