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If asked how you find an average, most people respond that you add up the values and divide by how many there were.
The average of 2, 2, 4, and 8 is 16/4 or 4.
But the average can also be found by saying 50% of the values were 2, 25% were 4 and 25% were 8 using the following calculation:
0.50*2 + 0.25*4 + 0.25*8 = 4
The latter method is best used for determining the average mass of an atom of one element. Many elements have a variety of isotopes that occur with frequencies that can easily be measured. Using % to determine the average is far superior to adding up each atom because such a sheer number of atoms exists even within a tiny sample.