Flexi Says: The atomic mass of an element is the weighted average of the atomic masses of the naturally occurring isotopes of that element. The weighted averages takes into account the percent natural abundances of each isotope. Use the atomic masses of each of the isotopes along with their percent abundances to calculate the average atomic mass.
Change each percent abundance into decimal form by dividing by 100. Multiply this decimal by the atomic mass of that isotope.
Add together the above value for each isotope to get the average atomic mass.