To begin with, assume that there is only one state at each energy level i (there is no degeneracy). What follows next is a bit of combinatorial thinking which has little to do in accurately describing the reservoir of particles. For instance, let's say there is a total of k boxes labelled a , b , … … , k . With the concept of combination, we could calculate how many ways there are to arrange N into the set of boxes, where the order of balls within each box isn’t tracked. First, we select N a balls from a total of N balls to place into box a , and continue to select for each box from the remaining balls, ensuring that every ball is placed in one of the boxes. The total number of ways that the balls can be arranged is