Answer: 2.81%
Explanation:
If the abundance of Fe-56 is 91.75%, then the other two isotopes have a combined abundance of 100%-91.75%=8.25%.
If we let the abundance of Fe-54 be x, then the abundance of Fe-57 is 0.0825-x. Substituting this into the atomic mass formula, we get:
[tex]55.85=(55.93)(0.9175)+(53.94)(x)+(56.94)(0.0825-x)\\55.85=51.315775+53.94x+4.69755-56.94x\\55.85=56.0133-3x\\-0.1633=-3x\\x=\frac{-0.1633}{-3} \approx 0.0544[/tex]
If x=0.1633/3, this means that 0.0825-x=0.0281, so Fe-57 is the rarest isotope, with an abundance of 2.81%