How do you find the average atomic mass?

Hint: As we know that the atomic mass is that mass of an element which tells us about how many times an atom of the element is heavier than that of the twelfth of an atom of carbon. We also know that the average of anything is calculated by taking the sum of each number divided by the total numbers.

Complete step-by-step answer:
As we have learnt that the atomic mass of an element is defined as the mass exactly equivalent to the one-twelfth the mass of one carbon$ - 12$ atom. Hydrogen being the lightest element was assigned a mass of $1$ and other elements were assigned masses relative to hydrogen.

Also, when we use atomic masses of an element in calculations, we actually use average atomic masses of elements because naturally occurring elements exist as more than one isotopes which are the atoms of the same element which possess different relative masses.

For instance: chlorine contains two atoms having relative masses as $35u$ and $37u$. The relative abundance of these isotopes in nature is in the ratio $3:1$. Thus the atomic mass of chlorine is the average of these different relative masses as described below:
Atomic mass of chlorine $ = \dfrac{{(35 \times 3) + (37 \times 1)}}{4} = 35.5u$

Therefore, in conclusion we can say that the average atomic mass of any element is defined as the ratio of sum of atomic masses of different isotopes of an element each multiplied by their natural abundance percentage to $100$.

Note:Remember that the atomic masses which are mentioned in the periodic table of elements for different elements actually represent their average atomic masses. And $'u'$ represents the unified mass or atomic mass unit.