Answer
78.920 u
Work Step by Step
There is only one other natural isotope, so the weighted average equation is:
(.4931)(80.916 u) + (1 - .4931)(atomic weight of unknown isotope) = 79.904 u
atomic weight of unknown isotope = 78.920 u
This answer makes sense; the average atomic weight is about 80, and the isotopes are about half and half in abundance, so if the known value is about 81, the unknown mass should be about 79.
