Most of the atomic masses on the Periodic Table are averages of the stable isotopes. A typical sample of a pure element has a certain percentage of each of the stable isotopes. The average mass is a weighted average based on these percentages. This presentation explains how a weighted average compares to the average computed by most students in the standard way. It also shows how to compute the average atomic mass from mas and percentage data.
Free worksheets are available to practice this skill on my website at http://evanschemistrycorner.com