[QUE/SM-04024] SM-PROBLEM

Consider a system having 3 non-interacting particles, each having three possible energy levels with energies $0\,,\,2\Delta\,,\,4\Delta$. Let $N_1\,,\,N_2\,,\,N_3$ be the number of particles occupying the energy levels Find the average of $N_1N_2$ when the system is at temperature $T$.