1. The problem statement, all variables and given/known data Carbon has two naturally occurring isotopes with the following isotopic masses: 12 (exactly) and 13.003. Calculate the percent natural abundance of carbon-13. 2. Relevant equations I'm not sure if this is right, but here's what I used. I set up a system of two equations, 12x+13.003y=12.0107 (12.0107 being the atomic weight for carbon, obtained from the periodic table) and x+y=1 (because these are the ratios, which will be used to find the percentages). 3. The attempt at a solution I solved for y, since that's the percentage I'm looking for. I don't think I should need to show my work, since it's pretty basic, but I was wondering if this is the right way to go about solving this problem. My answer came out to 1.1%, or 0.011 for y. Is this correct?