An easy and memorable way to teach the relationship between isotopes and average atomic mass is to calculate your grade point average (GPA). Just as each class grade contributes to your overall GPA, each atom in an element contributes to the element's average atomic mass. The atom or grade with the highest # frequency of occurrence (abundance) affects the average.
For example, 5A's, 1B, and 1C will give a grade that is close to the points of an A, which is 4.0. The average grade in this example is 3.57. The following formula can be used to get the average: [(# of grades) x (grade points)] + [(# of grades) x (grade points)] + .......... / (total # of grades) = GPA. This is because the grade of "A" has the higher # frequency which makes the GPA closest to it.
An atom's frequency or percent abundance works the same. The following formula is used: (% abundance) x (atom's mass) +........... = average atomic mass.
No need to divide by the total number of atoms. The % is changed to a decimal before you calculate.
Share if this makes it easier to understand.