What is the ratio by mass of hydrogen and oxygen in water?

The chemical formula of water is ${\text{H}}_2\text{O}$.Atomic mass of Hydrogen ($\text{H}$) $=1\text{u}$ Atomic mass of Oxygen ($\text{O}$) $=16\text{u}$ In a water molecule, there are 2 hydrogen atoms and 1 oxygen atom:
$$\text{Mass of Hydrogen}=2\times 1=2\text{u}$$
$$\text{Mass of Oxygen}=1\times 16=16\text{u}$$
The ratio by mass of Hydrogen to Oxygen is:
$$\text{Ratio}=\frac{2}{16}=\frac{1}{8}⟹1:8$$