What is the ratio by mass of carbon and oxygen in carbon dioxide?

The chemical formula of carbon dioxide is ${\text{CO}}_2$.Atomic mass of Carbon ($\text{C}$) = $12\text{u}$ Atomic mass of Oxygen ($\text{O}$) = $16\text{u}$ The mass of carbon in ${\text{CO}}_2$ is $12\text{u}$, and the mass of oxygen is $2\times 16=32\text{u}$.Therefore, the ratio by mass of carbon to oxygen is:
$$\text{Ratio}=\frac{12}{32}=\frac{3}{8}$$
Thus, the ratio is $3:8$.