Hello, Maile,
Compare the masses of each isotope to the average atomic mass. Normally, this would require some calculations, but look how close the average mass is to that of the the lowest isotope:
The difference between the lightest isotope and the average is only 0.004 AMU (atomic mass unit). Thus, the prevalent isotope must be the lightest, with very small amounts of the other two mixed in to account for the small increase in the average mass.
In fact, if the sample only had 1 % of the heaviest isotope, the average mass would become 16.015.
I hope this helps,
Bob
