Given:
Average atomic mass of Fe = 56 amu
Isotopes of Fe:
Fe54 - Atomic weight = 54 amu, abundance = ?
Fe57 - Atomic weight = 57 amu, abundance = 50%
Fe58 - Atomic weight = 58 amu, abundance = ?

Calculating percent abundance:
To find the percent abundance of Fe54 and Fe58, we can use the formula:
Average atomic mass = (abundance of isotope 1 x atomic weight of isotope 1) + (abundance of isotope 2 x atomic weight of isotope 2) + (abundance of isotope 3 x atomic weight of isotope 3) + ...

Substituting the given values, we get:
56 = (abundance of Fe54 x 54) + (0.5 x 57) + (abundance of Fe58 x 58)

Simplifying further:
56 = 54abundance of Fe54 + 28.5 + 58abundance of Fe58
27.5 = 54abundance of Fe54 + 58abundance of Fe58

Now we can solve for one variable in terms of the other. Let's solve for abundance of Fe54:
27.5 - 58abundance of Fe58 = 54abundance of Fe54
abundance of Fe54 = (27.5 - 58abundance of Fe58)/54

We know that the total abundance of all isotopes of an element is 100%. Therefore:
abundance of Fe54 + 50% + abundance of Fe58 = 100%
abundance of Fe54 + abundance of Fe58 = 50%

Substituting the expression for abundance of Fe54, we get:
(27.5 - 58abundance of Fe58)/54 + abundance of Fe58 = 0.5
27.5 - 58abundance of Fe58 + 54abundance of Fe58 = 27
-4abundance of Fe58 = -0.5
abundance of Fe58 = 0.125 or 12.5%

Therefore, the percent abundance of Fe54 is 37.5% (calculated as 100% - 50% - 12.5%).

Explanation:
The average atomic mass of an element is the weighted average of the atomic masses of all its isotopes, taking into account their relative abundance. In this case, we are given the atomic weights of three isotopes of Fe and we are asked to find the percent abundance of two of them. We use the formula for average atomic mass and solve for the unknown abundances by setting up an equation with two variables and two equations (one for the average atomic mass and one for the total abundance). We then solve for one variable in terms of the other and substitute in the total abundance equation to get the value of the other variable. The final answer is expressed as a percentage.