Percentage compostion of carbon in CaCO3

Ca = 40.08 amu

C = 12.01 amu

O = 16 * 3 = 48 amu

40.08 + 12.01 + 48 = 100.09 total amu

12.01 / 100.09 = 0.1199 * 100 = about 12% C

To find percent composition carbon you do

12.01 amu divided by the total amu of compound times 100 ...

amu is just the atomic mass number of the element/compound