To calculate the average atomic mass of an element you must find out the number of isotopes of that specific element and multiply it by the fraction of the mass. Isotopes are each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons. This causes them to differ in relative atomic mass but they do not differ in chemical properties.
Isotopes are always atoms from the same element. After successfully calculating average atomic mass several times you should get the hang of it quite well. A simple mathematical problem is all it takes.