A compound has 54.5 % carbon, 9.1 % hydrogen and 36.4 % oxygen…

$\%\text{C} = \dfrac{12}{44} \times \dfrac{m_{CO_2}}{m} \times 100$ and $\%\text{H} = \dfrac{2}{18} \times \dfrac{m_{H_2O}}{m} \times 100$. Dumas: $\%\text{N} = \dfrac{28 V}{22400\, m} \times 100$, with $V$ at STP in mL. Kjeldahl: $\%\text{N} = \dfrac{1.4\, M b V}{m}$. Carius: $\%\text{X} = \dfrac{\text{atomic mass}}{\text{molar mass of the product}} \times \dfrac{\text{mass of the product}}{m} \times 100$. Empirical formula: divide each per cent by the atomic mass, then by the smallest.