Gallium has an atomic mass of 69.723. The Ga-69 (68.926 amu) is 60.11%. What is the amu of the other isotope? I'd like to know the set up-thought it was (69.723)(0.6011) + (x)(0.3989)? But it isn't giving me the right answer.
The idea behind solving abundance or amu isotope problems is that each isotope contributes a fraction of its atomic mass - its abundance - to the average atomic mass.
So, in your particular case,
The set up you used is correct, since the abundance of the second gallium isotope is indeed equal to 1 - 0.6011 = 0.3989.
If you look up the actual atomic mass of
Finally, the result makes sense because the average atomic mass of gallium is closer to the lighter isotope,