The atomic mass of iron is 55.85 u.
The average atomic mass of Fe is the weighted average of the atomic masses of its isotopes.
We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its % abundance).
Thus,
0.0582 × 53.940 u = 3.139 u
0.9166 × 55.935 u = 51.270 u
0.0219 × 56.935 u = 1.247 u
0.0033 × 57.933 u = 0.191 u
________TOTAL = 55.85 u