Let the total atomic mass be the weighted average of the isotopes
Use the equation: average atomic mass = (mass of isotope 1 × abundance of isotope 1) + (mass of isotope 2 × abundance of isotope 2) + …
Let one isotope’s abundance be x
Let the other isotope’s abundance be 1 − x
Substitute the known isotope masses and the average atomic mass into the equation
Solve the equation for x
Convert x to a percentage by multiplying by 100
Convert 1 − x to a percentage by multiplying by 100
Check that the abundances add up to 100%
For more than two isotopes, set up one equation using the weighted average and use the given abundance relationships to solve for the unknowns