Most elements can naturally occur in multiple forms, or isotopes. The mass number for each isotope is the sum of numbers of protons and neutrons in the nucleus. Each proton and each neutron weigh 1 atomic mass unit (amu). The only difference between two isotopes of the same element is the number of neutrons per atom, which affects the atom's mass. However, the element always has the same number of protons.  The average atomic mass of the element takes the variations of the number of neutrons into account, and tells you the average mass per atom in a typical sample of that element. For example, the element silver (Ag) has two naturally occurring isotopes: Ag-107 and Ag-109 (or 107Ag and 109Ag). Isotopes are named after the "mass number," or the sum of protons and neutrons in one atom. This means Ag-109 has two more neutrons per atom than Ag-107, giving it slightly more mass. You'll need two pieces of information for each isotope, which you can look up in a reference book or an online source such as webelements.com. The first is the atomic mass, or the mass of one atom of each isotope. Isotopes with more neutrons have more mass.  For example, the silver isotope Ag-107 has an atomic mass of 106.90509 amu (atomic mass units). The isotope Ag-109 is slightly heavier with a mass of 108.90470. The last couple decimal places might be slightly different in different sources. Don't include any numbers in parentheses after the mass. The abundance tells you how common the isotope is, as a percentage of all atoms of the element. Each isotope contributes proportionally to its abundance (the more abundant the isotope, the more it will contribute to the average atomic mass). You can find this in the same source you found the mass. The abundances of all isotopes should add up to 100% (though it might be slightly off due to rounding errors).  The isotope Ag-107 has an abundance of 51.86%. Ag-109 is slightly less common with an abundance of 48.14%. This means that a typical sample of silver is 51.86% Ag-107 and 48.14% Ag-109....

TL;DR in Vietnamese:
Hiểu đồng vị và nguyên tử khối. Tìm khối lượng của mỗi đồng vị. Viết ra tỷ lệ tồn tại trong tự nhiên của mỗi đồng vị. Đổi tỷ lệ phần trăm đồng vị thành số thập phân. Tìm nguyên tử khối trung bình.