1. The problem statement, all variables and given/known data 1) "The relative atomic mass of antimony is 121.8. Antimony exists as two isotopes; antimony-121 and antimony-123. Calculate the relative abundances of the two isotopes." 2) "The relative atomic mass of rubidium is 85.5. Rubidium consists of two isotopes, rubidium-85 and rubidium-87. Calculate the percentage of rubidium-87 in naturally occurring rubidium." 2. Relevant equations N/A 3. The attempt at a solution I thought these questions should be relatively easy but the solutions at the back of the book don't seem to agree with my answers. For question 1, I did [121*x + 123*(100-x)]/100 = 121.8 and solved for x. Likewise for question 2. For question 1, I get that antimony-121 is 60% and antimony-123 is 40%, but the textbook says the answers should be 37.5% and 62.5% respectively. For question 2, I get Rb-85 occurring 75% and Rb-87 occurring 25%, yet the textbook says Rb-87 occurs 23.5%. However I checked my answers and I do get the average relative atomic masses that they state. Where have I gone wrong? Thanks.