calculate the percent composition, by mass, of carbon in Na2CO3. please explain how you get the answer.

Answer 1

%C = (mass C/total mass)*100 = ??
Just like any percent problem, it's the mass of the part you want over the total mass.
mass C = about 12
mass Na2CO3 = about 106 or so.
Where did those numbers come from? From the periodic table.

Answer 2

i got 45.8.

Answer 3

Are you implying that (12/106)*100 = 45.8%. No way. How do you get 45.8?