Atomic mass unit (amu):

One atomic mass unit (amu) is defined as a mass exactly equal to the one-twelfth mass of one C-12 atom.

Mass of one C-12 atom $= 12$ amu

$1$ amu $ =  \frac{\text{mass of one C-12 atom}}{12}$

A hydrogen atom is $8.40\%$ as massive as the standard C-12 atom, therefore relative atomic mass of hydrogen atom is

$=\frac{840}{100} \times 12$ amu $= 1.008$ amu

Similarly, relative atomic masses of O, Na, Al are 15. 9994 amu, 22.9898 amu, 26.9815 amu respectively.