Hello, Jaden,
We technically need a balanced equation for this reaction to be certain of the clculations. I'm unfamiliar with this reaction, but will assume it is a decomposition that realeases all the O atoms as O2 molecules.
This assumption would allow us to simply calculate how many grams of C3H6O3 we need to have 15 grams of oxygen.
Let's calculate the percentage of the total mass that is due to oxygen. For example, the molar mass of C3H6O3 is 90.03 g/mole. It has 3 O atoms per molecule, which add to (3 x 16.0 =) 48.0 AMU. The percentage of O in C3H6O3 is therefore (48.0 AMU/90.03 AMU), or 53.31%.
To find the mass required to contain 15.0 grams of O, use:
X*(0.5331) = 15 g [What mass times the O percentage produced 15g of O].
X = 37.48 grams of C3H6O3 contains 15.0 g of O.
Bob