Question: how to calculate the percent abundance of 2 isotopes?
Answer: Step 1: Find the Average Atomic Mass. Identify the atomic mass of the element from your isotopic abundance problem on the periodic table. ... 
Step 2: Set Up the Relative Abundance Problem. ... 
Step 3: Solve for x to Get the Relative Abundance of the Unknown Isotope. ... 
Step 4: Find percent abundance.