Question

If you have 100.0 grams of carbon and 100.0 grams or iron, which sample will contain more atoms

Answer 1

First you need to solve for the number of moles in each. 
1 mole of anything = atomic mas of that element (g)

Carbon has an atomic mass of 12.0107
1 mole of Carbon = 12.0107 g

Iron has an atomic mass of 55.845
1 mole of Iron = 55.845 g

now convert each from grams to moles 
(grams of Carbon cancel and you are left with moles of carbon)
100.0 g Carbon x 1 mol Carbon  = 8.326 mol Carbon
       12.0107 g Carbon

(grams of Iron will cancel and you are left with moles of iron)
100.0 g Iron x 1 mol Iron = 1.791 mol Iron
      55.845 g Iron

now we can use avogadro's number to solve for the amount of atoms in each 
1 mol = 6.02x10^23 atoms

8.326 mol Carbonx 6.02 x 10^23 atoms  = 5.012 x 10^24 atoms Carbon
       1 mol Carbon

1.791 mol Iron x 6.02 x 10^23 atoms  = 1.078 x 10^24 atoms Iron
       1 mol Iron

5.012 x 10^24 atoms Carbon > 1.078 x 10^24 atoms Iron