40 g of Ca = 1 mol

120 g = 120/40 = 3 mol

1 mol contains 6.022 x 10^23 atoms

Therefore,

3 mol contains 3 x 6.022 x 10^23 Ca atom

= 18.066 x 10^23 Ca atoms

56 g of Fe = 1 mol

120 g of Fe = 120/56 = 2.14 mol

1 mol contains 6.022 x 10^23 atoms

2.14 mol = 2.14 x 6.022 x 10^23 atoms

= 12.887 x 10^23 Fe atoms

Calcium has more no. of atoms.

Difference = 5.179 x 10^23 atoms