All about Relative atomic mass

The average weight of the isotopes relative to 1/12 the mass of a 6C12 isotope

Example:
The atomic mass of neon (Ne-19, amu 19.99245, natural abundance of 90.92%, Ne-20, amu 20.99396, natural abundance of 0.260%, Ne-21, amu of 21.99139. natural abundance of 8.82%)

Relative atomic mass of Ne: (19.99245× 0.9092)+(20.99396×0.00260)+(21.99139×0.0882) = 20.17 amu
Relative molecular mass: the sum of the atomic masses for each atom in the molecule
Example: H2O = (1× 2)+(1× 16) = 18