An unrooted tree with three species (a, b, and c. is shown. if we added a new species, d, anywhere in the phylogeny, we could find different unrooted species trees. suppose that you find another species, e, that might belong to any branch of the four-species tree. by adding species e anywhere in the phylogeny, we could find different unrooted species trees with the five species.

We could find that there are three different unrooted species trees.

The three species tree has three branches each leading from the internal node to one of the three tips. To create a four-species tree, we could add a new branch leading to new species.
D, to any of these three branches each point of attachment creates a different tree with species A, B, CA and D. There are three different unrooted four-species trees.
When we add another species, E, it will produce 15 possibilities. This is because each of those four-species trees has five branches which is three times five which is equal to fifteen.

Not sure what you mean by this question