In her science-fiction novel The Pride of Chanur and its sequels, author C. J.
Cherryh imagined an alien race, the stsho, which has three sexes instead of
two as we humans do. Thus, each stsho has three parents, which will be
referred to as X, Y, and Z instead of "mother" and "father." Here is the
beginning of a family tree for a certain stsho, named Tle-nle, showing gtst
three parents, gtst grandparents (G), and gtst great-grandparents (GG).
How many ancestors does Tle-nle have, going back through the fifth
generation of gtst family tree, counting Tle-nle gtst-self as the first
generation? When calculating the sum of the appropriate geometric series,
include the term 1 for Tle-nle gtst-self in your total.