Use the average atomic mass equation:
average atomic mass = (fraction of isotope 1 × mass of isotope 1) + (fraction of isotope 2 × mass of isotope 2) + …
Let the unknown isotope abundance be x
Let the other isotope abundance be 1 − x
Substitute the known atomic masses and the average atomic mass into the equation
Solve the equation for x
Convert x to a percentage by multiplying by 100
If there are more than two isotopes, set up one variable for each unknown abundance
Use the fact that all isotope abundances must add up to 1
Solve the system of equations for the unknown abundances