Answer
20.18
Explanation:
The atomic mass of X is the weighted average of the atomic masses of its isotopes.
We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its percent of the total).
Set up a table for easy calculation
0.9048 × 19.99 u = 18.087 u
0.0027 × 20.99 u = 0.0567 u
0.0925 × 21.99 u = 2.0341 u
TOTAL = 20.18 u
