Isotopes
Atoms with different numbers of neutrons are called isotopes. For instance, carbon has fifteen known isotopes, but only three are commonly referenced: carbon-12, carbon-13, and carbon-14.
All three isotopes have six protons (carbon's atomic number), but C-12 has 6 neutrons, C-13 has 7 neutrons, and C-14 has 8 neutrons.
All three isotopes have six protons (carbon's atomic number), but C-12 has 6 neutrons, C-13 has 7 neutrons, and C-14 has 8 neutrons.
Average Atomic Mass & Percent Abundance
Here are two videos from ChemisNATE that shows a quick and easy way to calculate average atomic mass. The second video shows how to calculate percent abundance.