Hi! I need help with the following question: A system has two energy levels, ε and ε1 that can be occupied by fermions (spin=1/2) that are non-interacting from a reservoir at temperature T and chemical potential μ. Compute the avarage occupation number of the state with energy ε. I have written down the grand canonical partition function and it is (1+e^(-β(ε-μ))^2)((1+e^(-β(ε1-μ)))^2) and I know the formula for avarage occupation number for fermions is ΣnP(n) where n goes from 0 to 1. But then I'm stuck. The hint I got was that there can be up to four particles and there are different possibilities to put these into the energy levels. Very thankful for any help!