There are two stable isotopes of Gallium Their masses are 68.92558 and 70.9247050 AMU. If the average atomic mass of gallium is 69.7231, what is the natural abundance of the lighter isotope?

Question

I don't know where to start!

Answer 1

Let x = fraction of the lighter isotope.
1-x = fraction of the other isotope.
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x(lighter AMU) + (1-x)(other isotope)=avg.
Substitute and solve for x.