A sample of pure carbon dioxide, irrespective of its source contains 27.27% carbon and 72.73% oxygen. The data support

The ratio by mass of carbon and oxygen present in CO₂ = $\frac{12}{2\times 16}=\frac{3}{8}$

From given percentages, 

The ratio by mass of carbon and oxygen present in CO₂ = $\frac{27.27}{72.73}=\frac{3}{8}$

Carbon dioxide prepared by any method contains carbon and oxygen in the same proportion by mass. Thus, we can say that the given data proves the law of constant proportions. The Law of constant proportion is also known as the law of definite proportion.