1. The problem statement, all variables and given/known data The atomic weight of bromine is 79.904. The natural abundance of Br-81 is 49.31%. What is the atomic weight of the only other natural isotope of bromine? Z = 35 2. Relevant equations The answer is 78.92 amu (Br-79) and Br-81 happens to be 80.92 amu. 3. The attempt at a solution Restated, the problem is: 0.4931y + 0.5069x = 79.904 ...which amounts to a linear equation, not a single solution for x. I feel like either I've overlooked some facts/method to derive y (Br-81 amu), or there's some math maneuvering I've forgotten. Before this semester, I was 12 years removed from a classroom -- I have gotten some practice in, but the latter wouldn't surprise me. Thanks for taking the time.