Determine the mass fractions of chemical elements in marble CaCO3

Knowing the atomic masses of the elements, one can determine their proportions by mass in a particular substance. This will be the ratio of the atomic masses of the element, multiplied by the number of atoms in the molecule, by the total mass of the molecule.
For CaCO3 we have:
M Ca = 40 amu,
M C = 12 amu,
M O = 16 amu, then the mass fractions are equal:
D (%) Ca = 40: (40 + 12 + 16 * 3) * 100% = 40: 100 * 100% = 40%.
D (%) C = 12: 100 * 100% = 12%.
D (%) O = 16 * 3: 100 * 100% = 48%.