Question: how to calculate the percentage of two isotopes?
Answer: As a percent, the equation would be: (x) + (100-x) = 100, where the 100 designates the total percent in nature. If you set the equation as a decimal, this means the abundance would be equal to 1. The equation would then become: x + (1 – x) = 1. Note that this equation is limited to two isotopes.