Identify the isotopes and their masses
Let the percent abundance of one isotope be x
Let the percent abundance of the other isotope be 100 – x
Use the average atomic mass equation:
average atomic mass = (mass of isotope 1 × abundance of isotope 1) + (mass of isotope 2 × abundance of isotope 2)
Convert percent abundances to decimals before substituting
Solve the equation for x
Convert the decimal abundance back to a percentage
Check that all abundances add up to 100%