The example used:

Calculate the percent Na, S and O in the compound Na₂SO₄.

To start the calculation, assume 1 mole of Na₂SO₄.  Set up a table and make the calculations as follows:

element	moles	molar mass /g mol-1	mass* /g	percent**
Na	2	23.0	46.0	32.4 %
S	1	32.0	32.0	22.5 %
O	4	16.0	64.0	45.1 %
total			142.0

*From the definition of molar mass:
m = nM

** percent definition: