need help showing my work and solving these problems please!!

Solve the following equations: 3. Percent Composition from Mass Problem - When a 13.60g sample of a compound containing only magnesium and oxygen is decomposed, 5.40g of oxygen is obtained. What is the percent composition of this compound? Answer 4. Percent Composition from a Chemical Formula Problem - Propane (C3H8), the fuel commonly used in gas grills, is one of the compounds obtained from petroleum. Calculate the percent composition of propane. Answer 5. Calculating Mass using Percent Composition Problem - Calculate the mass of carbon and the mass of hydrogen in 82.0g of propane